Characterisation of Blends Based on Hydroxyethylcellulose and Maleic Acid-alt-Methyl Vinyl Ether

Characterisation of Blends Based on

Hydroxyethylcellulose and Maleic

Acid-alt-Methyl Vinyl Ether

Olga V. Khutoryanskaya,1 Vitaliy V. Khutoryanskiy,2 Richard A. Pethrick*1

1 University of Strathclyde, Department of Pure & Applied Chemistry, University of Strathclyde, Thomas Graham Building,295 Cathedral Street, Glasgow, G1 1XL, UKE-mail: [email protected]

2 University of Manchester, School of Pharmacy and Pharmaceutical Sciences, Coupland III, Coupland Street, Manchester,M13 9PL, UK

Received: February 15, 2005; Revised: May 25, 2005; Accepted: May 30, 2005; DOI: 10.1002/macp.200500069

Keywords: cellulose ether; hydrogen bonding; hydrophilic polymers; polymer-polymer miscibility; water-soluble polymers

Introduction

Polymeric blends based on water-soluble polymers are of

considerable interest because they are relatively easy to

prepare and have a variety of potential uses including bio-

medical and pharmaceutical applications,[1–3] as polymer

membranes,[4,5] coatings and packaging films,[6] and in

electronics.[7] Miscibility of polymers in a blend are drama-

tically influenced by the presence of specific interactions;

hydrogen bonding and electrostatic interactions[8,9] being

the most important. Hydrogen bonding is responsible for

miscibility in a number of different systems containing

poly(carboxylic acids) and non-ionic polymers. Specific

interactions have also been reported in blends between

poly(acrylic acid) (PAA) and various water-soluble poly-

saccharides.[3,10–15] Complete miscibility as a rule is attri-

buted to the strong intermacromolecular hydrogen bonding

between the component polymers. Studies on blends of

water-soluble polysaccharides and carboxylic acid copoly-

mers containing the groups, which are not able to form

hydrogen bonds, are quite limited. Bumbuet. al. have report-

ed studies on blends based on maleic acid-vinyl acetate

Summary: Hydrophilic polymeric films based on blends ofhydroxyethylcellulose and maleic acid-co-methyl vinyl etherwere produced by casting from aqueous solutions. The phy-sicochemical properties of the blends have been assessedusing Fourier transform infrared spectroscopy, thermal gravi-metric analysis, differential scanning calorimetry, dielectricspectroscopy, etc. The pristine films exhibit complete misci-bility due to the formation of intermacromolecular hydrogenbonding. The thermal treatment of the blend films leads tocross-linking via intermacromolecular esterification andanhydride formation. The cross-linked materials are able toswell in water and their swelling degree can be easily con-trolled by temperature and thermal treatment time. The for-mation of the crosslinks is apparent in the dynamic propertiesof the blends as observed through the mechanical relaxationand dielectric relaxation spectra. The dielectric character-istics of the material are influenced by the effects of change inthe local structure of the blend on the ionic conductionprocesses and the rate of dipolar relaxation. Separation ofthese processes is attempted using the dielectric modulusmethod. Significant deviations from a simple additive rule ofmixing on the activation energy are observed consistent with

hydrogen bonding and crosslinking of the matrix. This paperindicates a method for the creation of films with good mech-anical and physical characteristics by exposing the blends to arelatively mild thermal treatment.

Scheme of hydrogen bonding between hydroxyethylcellu-lose and maleic acid-co-methyl vinyl ether.

Macromol. Chem. Phys. 2005, 206, 1497–1510 DOI: 10.1002/macp.200500069 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper 1497

(MA-VA) copolymer and dextran,[16] pullulan[17] and

hydroxypropylcellulose.[18] This copolymer consists of

alternating MA units with two carboxylic groups and vinyl

acetate units, which are not active in formation of hydrogen

bonds. The presence of intermacromolecular hydrogen

bonding and the formation of ester bonds lead to com-

patibility at 85–95 wt.-% of dextran in the blend, but lower

compositions of dextran are immiscible and phase separa-

tion is observed.[16] The blends containing greater than 85%

of pullulan are homogeneous, whereas the blends with lower

amount of polysaccharide show distinct phase separa-

tion.[17] The blends of MA-VAwith HPC containing 10–30

and 70–90% of the polysaccharide exhibit single Tg values

and are transparent, supporting the assumption of their

complete miscibility.[18] The films containing 60% of HPC

did not show a glass transition and the blend containing 50%

HPC could not be produced because of precipitation of

interpolymer complex of MA-VA/HPC. The miscibility of

the blends was explained in terms of the balance of hydro-

gen-bonding and hydrophobic interactions between the

component polymers. A similar hypothesis was previously

used to interpret complexation of dextran and HPC with

respect to PAA.[3] Indeed, HPC exhibits much a higher

ability to interact with PAA via hydrogen bonding and

hydrophobic interactions than dextran, which justifies the

poorer ability of the latter polymer to form miscible blends

with MA-VA.

This paper reports studies on blends based on hydroxy-

ethylcellulose (HEC) and an alternating copolymer of

methyl vinyl ether and maleic acid (PMVEMAc). PMVE-

MAc is a water-soluble copolymer containing MA units

with two carboxylic groups, which can act as proton-donors

and form hydrogen bonds with hydroxyl and ether groups of

HEC. The second component of this copolymer is methyl

vinyl ether (MVE) can also act as proton-acceptor with

respect to the hydroxyl groups of HEC. The purpose of this

work was to prepare the polymeric films based on HEC-

PMVEMAc, to assess the miscibility of the polymers in the

blends and to evaluate the possibility of the polymers cross-

linking.

Experimental Part

Materials and Film Preparation

Poly[(methyl vinyl ether)-alt-(maleic acid)] (Mw � 1.98�106 Da) and hydroxyethylcellulose (DS¼ 1, MS¼ 2, Mw �2.5� 105 Da) were supplied by Sigma-Aldrich and usedwithout further purification. The binary polymeric blends wereobtained by mixing PMVEMAc and HEC aqueous solutions(2 wt.-%) in different proportions and stirring at roomtemperature for 30 min. The films were prepared from aqueoussolutions of pure polymers and their blends, by casting onpolystyrene Petri dishes. After evaporation of solvent (water)over 2–3 d at room temperature the films were stored in a desic-cator. All films had a thickness of approximately 0.04–0.06 mm.

Thermal Treatment

The thermal treatment of the film has been performed in atemperature-controlled oven under vacuum.

Determination of Swelling Degree

The degree of the films swelling (DS) in water was gra-vimetrically determined by the following formula:

DS ¼ ðm� m0Þ=m0 ð1Þ

where m and m0 are the weights of the film sample in swollenand dry state, respectively. The experiments were performed intriplicates and the average DS was calculated and reported.

Modulated Temperature Differential ScanningCalorimetry (MTDSC)

MTDSC studies were performed on a TA Q1000 over atemperature range from�80 to 250 8C, under an atmosphere ofdry nitrogen at a flow rate of 20 mL/min. The samples weretypically between approximately 1–2 mg and 4.7–5.3 mg. Thesamples were first heated from 30 to 70 8C at a heating rate of5 8C/min and held for 60 min isothermally, and then cooled to�80 8C at a rate of 10 8C/min and then heated again at the samerate to 250 8C.

Thermogravimetric Analysis (TGA)

TGA was performed on a Shimadzu ThermogravimetricAnalyzer (TG-51) in nitrogen atmosphere at a heating rate of10 8C/min and a temperature range from room temperature to500 8C. The nitrogen flow rate was 20 mL/min.

Fourier Transformed Infrared (FTIR) Spectroscopy

FTIR spectra of the samples were recorded using KBr discswith a Perkin-Elmer spectrometer at a resolution of 4 cm�1

with 10 scans.

Dielectric Relaxation Spectroscopy

The dielectric properties of the films were measured using theSI1260 Impedance/gain phase analyzer (Solartron) and Novo-control system (Novotronic (Novocontrol GmbH), Broadbanddielectric converter and Microtronic). The frequency and tem-perature range were 0.1–1 MHz and 25–70 8C, respectively.The time of temperature stabilisation was 10 min. Electrodediameter was 9.985 mm. Heating rate was 10 8C/min. Themethod used has been described previously.[19]

Results and Discussion

All the films prepared from HEC and PMVEMAc, as well as

their blends, were optically transparent. Pure HEC film

exhibited high elasticity; however as the PMVEMAc

content was increased so the films of the blend became

brittle. All the films dried on air without thermal treatment

were soluble in water.

1498 O. V. Khutoryanskaya, V. V. Khutoryanskiy, R. A. Pethrick

Macromol. Chem. Phys. 2005, 206, 1497–1510 www.mcp-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Infrared Spectroscopy

The FTIR spectra were recorded for the films based on

PMVEMAc, HEC as well as their blends dried in three

different regimes: dried on air at room temperature

(Figure 1), heated at 70 8C for 24 h (Figure 2) and heated

to 200 8C with 10 8C/min and then cooled to a room

temperature (Figure 3). The bands characteristic for OH-

stretching vibrations of PMVEMAc are observed at 3 553,

3 455 and 3 636 cm�1 for air-dried sample, the film treated

at 70 8C for 24 h and the sample heated up to 200 8C,

respectively. The shift of this band upon thermal treatment

at 70 8C is attributed to the loss of some of the residual

solvent water molecules bound by hydrogen bonding with

carboxylic groups of PMVEMAc. The assignment of this

band to this interaction is in agreement with the data of

Rocco and co-workers for pure PMVEMAc.[19] The

dramatic shift of this band after the thermal heating until

200 8C is likely due to the dehydration of the carboxylic

groups of the copolymer with the reformation of anhydride

cycles. The two shoulders at 1 846 and 1 779 cm�1 typical

for maleic anhydride cycles observed in the spectrum of

PMVEMAc powder disappear completely in the spectrum

of the air-dried film due to the hydrolysis of anhydride

cycles into COOH groups in the presence of water in course

of casting process. The film dried under relatively mild

conditions at 70 8C, shows these bands again at 1 849 and

1 781 cm�1 indicating reformation of the anhydride struc-

ture. These changes to the anhydride are also reflected in the

shifts of the band at 1 185 cm�1 in the PMVEMAc powder

Figure 1. FTIR spectra of air-dried HEC, PMVEMAc and their blends.

Characterisation of Blends Based on Hydroxyethylcellulose and Maleic Acid-alt-Methyl Vinyl Ether 1499


to 1 191 cm�1 in the air-dried film and to 1 188 cm�1 in the

thermally treated film. Another important feature of

the PMVEMAc spectra is the presence of the band at

1 092 cm�1 assigned to the C-O-C stretching vibration of

MVE units. The PMVEMAc film heated to 200 8C(Figure 3) shows the presence of the bands at 1 860 and

1 779 cm�1 assigned to the presence of anhydride bonds and

the intensity of these bands is markedly increased. The

band at 1 779 cm�1 for this film is no longer a shoulder but

has become quite a strong peak, which indicates that most of

the free carboxylic groups of PMVEMAc have been trans-

formed into anhydrides under these conditions. The band at

1 184–1 191 cm�1 observed in the spectra of PMVEMAc

powder, air-dried film and the sample dried at 70 8C is

shifted to 1 225 cm�1 upon heating up to 200 8C, which also

consistent with anhydride formation in the material.

The FTIR spectrum of pure HEC is consistent with the

data earlier reported by Langkilde and Svantesson.[20] For

air-dried pure HEC film we can observe the shift of the OH-

stretching vibration band at 3 445 cm�1 found for the HEC

Figure 2. FTIR spectra of HEC, PMVEMAc and their blends (thermal treatment 24 h at70 8C).



powder to 3 427 cm�1 and appearance of a new shoulder

around 3 200 cm�1, which may be ascribed to hydrogen

bonding of OH-groups with water molecules. The HEC film

treated thermally at 70 8C shows this band at 3 434 cm�1

and the sample heated up to 200 8C at 3 439 cm�1. In air-

dried HEC film, there is a band at 1 635 cm�1 due to water

molecules absorbed in the amorphous regions of the poly-

mer.[21,22] After thermal treatment of the film sample both at

70 8C and at 200 8C, this band is observed at 1 641 and

1 639 cm�1, respectively, and additionally a new band at

1 728–1 743 cm�1 appears. The latter band can be associ-

ated with the presence of small amount of COOH-groups,

which is formed as a result of oxidation of the OH-groups at

higher temperatures.

All the characteristic bands of the component polymers

are present in the spectra of their air-dried blends and the

intensities of the bands vary depending on their composi-

tion in the mixture. The OH-stretching vibration band at

3 553 cm�1 observed in air-dried PMVEMAc undergoes a

decrease upon addition of HEC. For example, for the blend

containing 50% of HEC it appears at 3 449 cm�1, the

blend with 70% of HEC shows this band at 3 442 cm�1, the

blend with 90% of HEC at 3 417 cm�1. In the blends treated

thermally at 70 8C this trend is also observed. Such a

shifting of this band in the blends may be a consequence of

hydrogen bonding between the components and confirms

their compatibility. The blends treated thermally at 70 8Cshow the presence of the bands at 1 776–1 779 cm�1 and

Figure 3. FTIR spectra of HEC, PMVEMAc and their blends (heated up to 200 8C).



1 843–1 848 cm�1 are observed, which can be associated

with the formation of anhydride groups. In the blends

treated thermally at 200 8C these bands are situated at

1 765–1 779 and 1 826–1 846 cm�1. The appearance of the

new band at 1 172–1 177 cm�1 in the spectra of thermally

treated blends is the evidence of esterification reaction

between carboxylic groups of PMVEMAc and OH-groups

of HEC. Similar new band at 1 168–1 170 cm�1 was

observed by Bumbu and co-workers for the blends of maleic

acid-alt-vinyl acetate with hydroxyethylcellulose.[18]

Thermogravimetric Analysis

The thermal stability of the films of pure polymers, as well

as their blends was investigated over the temperature

range 25–500 8C (Figure 4). The degradation profile of pure

PMVEMAc film exhibits four main stages. The first stage

associated with a weight loss of up to approximately 10%

starts at 65 8C and is related to evaporation of water loosely

bound to the hydrophilic domains within the material. The

second stage of the weight loss which can be up to 10–12%,

starts at approximately 100 8C and the rate of this process

reaches maximum at 165 8C. This feature appears to be

related to the dehydration of PMVEMAc carboxylic groups

leading to the formation of anhydride and is confirmed by

the FTIR studies. The third and fourth stages are related to

phenomena occurring in the range of 200–500 8C and

associated with a weight loss of about 50%. In this region a

number of complex degradation processes will occur

associated with solid state transformations which occur

prior to loss of low molecule mass fragments.[23] Chung and

co-workers[24] have reported the thermal degradation of

poly[(methyl vinyl ether)-alt-(maleic acid)] for up to 400 8Cand their results are agreed well with our observations.

The thermal decomposition of HEC has been described

by Li et al.[25] Although our results resemble their data in

general some difference can be observed, which is probably

caused by batch-to-batch variation of hydroxyethylcellu-

Figure 4. Thermogravimetric analysis of PMVEMAc, HEC and their blends.



lose. The weight loss curve of our HEC sample is char-

acterised by two processes. Starting at 60 8C, a decrease in

sample mass of�8% is observed, caused by the elimination

of unbound water. The weight of the sample is practically

constant between 60 8C and 200 8C, but the further increase

in temperature results in significant weight loss and occur-

rence of two subsequent decomposition processes, which is

clearly seen from the derivative weight curve. The amount

of char obtained at 400 8C is about 25%, in a good

agreement with earlier data.[25]

The thermal behaviour of the blends is more complex and

depends on the composition of the blend. Starting at approx-

imately 50 8C all blends lose up to 10% of mass, which is

due to the evaporation of water loosely bound to the

hydrophilic domains. An increase of PMVEMAc content in

the blend from 10 to 20% leads to a partial suppression

of the double thermal feature characteristic of the HEC

that has the maximum rate of weight loss at 293 8C. The

blends with PMVEMAc content from 30 to 80% exhibit the

major thermal effects in the temperature regions 195–

275 8C and 270–420 8C, which represent the degradation

features of the synthetic copolymer and HEC, respectively.

However the thermal effect typical for pure PMVEMAC at

about 250 8C is observed at much lower temperature in the

blends (for example, at about 225 8C for the blend

containing 40% of PMVEMAC). All these changes in the

degradation profile of the blends compared to the pure

polymers indicate on the presence of specific interactions

between the components, which confirms their miscibility

in the blend.

Swelling of Thermally Cross-Linked Films

The possibility of cross-linking of HEC, PMVEMAc as

well as their 50:50% blend upon thermal treatment has been

studied by determination of their solubility or swelling

characteristics in water. Table 1 summarises the results

obtained for thermal treatment of the films at 80 8C, 100 8Cand 150 8C for 1, 2 and 3 h. It can be clearly seen that neither

pure HEC nor pure PMVEMAc films undergo cross-linking

upon thermal treatment at 80 8C and 100 8C for 1–3 h. The

cross-linked materials based on pure HEC can be only

prepared by the thermal treatment of the films at 150 8C.

The equilibrium degree of swelling of cross-linked HEC is

relatively low, which is due to the non-ionic nature of the

polysaccharide and its chain stiffness. The thermal treat-

ment of PMVEMAc films at 150 8C results in cross-linking

and the materials obtained exhibits a high swelling capacity,

which gradually decreases from 664.2 to 198.5 g of water

per 1 g of dry polymer upon increase in the thermal

treatment time from 1 to 3 h. The high swelling ability of

PMVEMAc hydrogels is due to the presence of carboxylic

groups and the flexibility of its chains.

The blending of PMVEMAc and HEC facilitates the

cross-linking reactions caused by thermal treatment. The

cross-linked films can be prepared even by thermal treat-

ment at 80 8C and 100 8C, which is probably due to the

occurrence of an intermacromolecular esterification reac-

tion between carboxylic groups of PMVEMAc and hydrox-

yl groups of HEC. Similar thermal cross-linking has been

reported for the blends of poly(acrylic acid) with methyl-

cellulose,[14] hydroxyethylcellulose,[26] and poly(2-hydr-

oxyethyl vinyl ether).[27] The variation in temperature as

well as the thermal treatment time allows preparing the

hydrogel films with the required degree of swelling

characteristics.

Differential Scanning Calorimetry

As was discussed above, the carboxylic groups of

PMVEMAc undergo anhydride formation at relatively

low temperatures, which makes the determination of the

exact glass transition point (Tg) very difficult. The literature

contains conflicting data on the glass transition of both

PMVEMAc and its anhydride form (PMVEMA). Chung

and co-workers have reported that within experimental

error the Tg of PMVEMA increases with molecular weight

and levels off at about 153 8C.[24] The Tg of PMVEMAc is

about 10 8C less than the anhydride form. The authors attri-

bute this difference to the higher stiffness of five-membered

cycles in anhydride form in comparison with PMVEMAc.

Also it would be expected that any traces of water in the

sample could decrease the Tg significantly due to plasticisa-

tion of the hydrophilic polymers.[28,29] PMVEMAc is

hygroscopic and cannot be fully dried at room temperature.

Hence the presence of water can decrease the Tg of

PMVEMAc, which was probably the explanation for a Tg

reported by Rocco and co-workers, of �9 8C.[30]

DSC of the pure polymers and their blends after isother-

mal treatment of the samples at 70 8C for 30 min (Figure 5)

show a clear feature which can be associated with the Tg

Table 1. Swelling of thermally cross-linked pure PMVEMAc,HEC films and their blend.

Sample Temperature Degree of swelling

8C 1 hthermal

treatment

2 hthermal

treatment

3 hthermal

treatment

HEC 80 a) a) a)

100 a) a) a)

150 8.9 7.3 7.2PMVEMAc/HEC

(50:50 wt.-%)80 1.18 0.86 0.73

100 0.78 0.52 0.47150 0.20 0.18 0.17

PMVEMAc 80 a) a) a)

100 a) a) a)

150 664.2 240.6 198.5

a) Dissolved.



process. However, we have to admit that this feature may be

significantly affected by a dehydration process of the carbox-

ylic groups discussed earlier. Pure PMVEMAc shows

the thermal effect at 149 8C, which is close to the Tg value

reported by Chung and co-workers.[24] The Tg value of pure

HEC is observed at 97 8C. The Tg values for the polymers

and their blends are also presented in Table 2. It can be seen

that the blends show the presence of a single Tg, which is in

between the thermal transitions of the pure components.

This is an additional proof of the components complete

miscibility. Slightly different behaviour is observed for the

samples held isothermally for 60 min. In the 80, 70 and 60%

blends, holding the films isothermally for 60 min generates

a higher temperature peak, which can be ascribed to the

formation of the intermacromolecular esterification reac-

tion between carboxylic groups of PMVEMAc and hydrox-

yl groups of HEC constraining the motion of the polymer.

Changes in the peak profile and location are consistent with

the effects of water loss and change of the environment with

polymer composition. Crosslinking to form a polymer net-

work and loss of plasticizing due to water leads to an

increase in theTg. The main transition moves consistently to

higher temperature consistent with loss of water increasing

the Tg and the appearance of the higher temperature feature

reflects the effects of crosslinking.

Dynamic Mechanical Thermal Analysis

In order to further clarify the nature of the thermal transi-

tions in HEC, PMVEMAc and their blends dynamic mech-

anical thermal analysis (DMTA) was performed. Before the

analysis was performed, the air-dried films were thermally

treated in the oven at 50 8C for 3 d to remove any traces of

water. Figure 6 shows the dependence of modulus and

damping factor for the film of pure HEC. It can be clearly

seen that the film exhibits the thermal transitions at about

35 8C and 115 8C. The existence of two glass transitions in

the films based on blends of poly(acrylic acid)/poly(2-

hydroxyethyl vinyl ether) as well as pure poly(acrylic acid)

has been previously explained on the basis of the materials

containing plasticised regions and non-plasticised re-

gions.[31] Taking into account that the pure HEC has a very

good ability to retain water even the sample dried at 50 8Cfor 3 d shows two glass transitions, typical for plasticised

and unplasticised HEC. The first glass-transition could be

related to a glass transition typical for samples containing

water traces. Since the samples were analyzed in open-air

conditions, and water traces are eliminated between 50–

100 8C; thereafter the second glass transition, typical for dry

samples, is observed. In order to confirm this hypothesis,

the HEC sample which had been previously dried at 50 8Cfor 3 d was left in air for a further 3 d assuming that it will

absorb some water. It can be clearly seen from Figure 7, that

the first glass transition for this sample is observed at 70 8Cand it is much more pronounced. The second glass

transition of this sample is observed at 104 8C as a shoulder

on the damping factor curve, which may indicate that even

in these conditions the sample still contains some traces of

water compared to the samples shown on Figure 6. Our

results on the glass transition of pure HEC are in a good

Figure 5. DSC curves of PMVEMAc, HEC and their blends; [a]Equilibration at 70 8C and held isothermally for 30 min,periodicity 4 8C and amplitude 100 s; [b] Equilibration at70 8C and held isothermally for 60 min, periodicity 0.5 8C andamplitude 100 s.

Table 2. Glass transition temperatures of HEC, PMVEMAc andtheir blends. [a] Equilibration at 70 8C and held isothermally for30.00 min, [b] Equilibration at 70 8C and held isothermally for60 min.

Sample Tpeak [a] Tpeak [b]

8C 8C

HEC 100% 97 –HEC 90% – –HEC 80% 129 149HEC 70% 139 149HEC 60% 141 148HEC 50% 143 149HEC 40% 139 160HEC 30% 141 148HEC 20% 145 160HEC 10% 149 150PMVEMAc 100% 149 152



agreement with the data reported in the review of

Doelker.[32]

The DMTA analysis of the pure PMVEMAc reveals the

existence of the glass transition at 112 8C. The DMTA data

for the blends shows the presence of a single glass transi-

tion, which is an indication on complete miscibility be-

tween the components. It should be noted that the first glass

transition within the range 35–70 8C typical for pure HEC

and caused by plastification effect is not observed for

PMVEMAc and its blends with HEC. It is likely that the

intermacromolecular hydrogen bonding between HEC and

PMVEMAc not only ensures the complete miscibility be-

tween these polymers but also blocks some of HEC func-

tional groups that previously had the ability to retain water

molecules. The effect of intermacromolecular hydrogen

bonding can be also seen from the modulus values, which

for the blends are significantly higher than for the films

consisting of the pure polymers (Figure 6).

Dielectric Analysis

The local mobility and the nature of the polymer-polymer

interactions can be probed using dielectric relaxation. The

mobility of a dipole will be influenced by short range effects

such as local hydrogen bond interactions, and the influence

of crosslinks restricting cooperative larger scale motions of

the chain backbone. In addition, in a system which contains

ionisable groups; carboxylic acids, moisture etc. conduc-

tion through the matrix can be modulated by trapping of

charge at internal interfaces and electrodes. The morphol-

ogy of the polymer will influence the breadth of the dipolar

process and the mobility of free charge carriers. The

changes in the chemical composition and the structural state

alter the electric properties of the system, namely:[33]

(i) the dc conductivity depends critically on the mobility

of water molecules within the matrix and will be

influenced by the number of binding sites and crosslink

density.

(ii) the dipole relaxation will reflect both local thermally

activated and also longer range free volume limited

relaxation processes.

Thus the complex permittivity is a function of both

frequency and temperature and can be generally expressed

by:

e*ðo; TÞ ¼ e0ðo; TÞ � e00ðo; TÞ ð2Þ

where the dielectric permittivity, e0, and loss, e00, contain

contributions from both the dipolar polarisation and dc

conduction, T is the temperature. When there is relatively

low ionic concentration and therefore, low dc conductivity,

then

e0ðo; TÞ ¼ e0dipðo; TÞ ð3Þ

Figure 6. DMTA analysis of PMVEMAc, HEC and their blends.

Figure 7. DMTA analysis of pure HEC sample dried at 50 8C for3 d and then left in air for a further 3 d.



and

e00ðo; TÞ ¼ e00ionðo; TÞ þ e00dipðo; TÞ ð4Þ

where the subscripts ‘dip’ and ‘ion’ refer to the contribu-

tions from dipolar polarisation and dc conductivity, res-

pectively. The contribution to the dielectric loss from dc

Figure 8. Dielectric Permittivity and Loss plots as a function of frequency for HEC andPMVEMAc and their blends over the temperature range 25 8C to 70 8C.



conductivity, s, is given by

e00ion ¼ soeo

ð5Þ

At low frequencies, the contribution to the dielectric

loss from dipolar orientation is negligible. Studies of the

dielectric relaxation of HEC and PMVEMAc are presented

in Figure 8. All the systems show a very large low frequency

permittivity and loss behaviour and are typical of hydrated

polymer systems. In addition to processes attributed to ionic

conduction, features can be observed as a result of

microphase separation and ionic trapping within the matrix.

Figure 9. Plots of the real and imaginary parts of the dielectric modulus for HEC,PMVEMAc and their blends over the temperature range 25–70 8C.



The HEC can form ordered regions at a nano scale and these

will generate interfacial polarisation effects. In order to

separate the various contributions the data are transformed

into the modulus representation, Figure 9. The measured

values for the dielectric permittivity, e0, and the dielectric

loss, e00, may be transformed into the complex electric

modulus, M*, using the following equations:

M* ¼ 1

e*¼ M0 þ iM00 ð6Þ

where

M0 ¼ e0

e02 þ e002ð7Þ

and

M00 ¼ e00

e02 þ e002ð8Þ

where M0 and M00 are the real and imaginary components of

the complex electrical modulus. WhenM* is entirely due to

ionic conduction it is related to conductivity relaxation

time, ts, by the equation

M*ðo; TÞ ¼ Ms

iotsðTÞ1 þ iotsðTÞ

� �ð9Þ

where

ts ¼eoes

sð10Þ

and

Ms ¼ e�1s ð11Þ

also M0 and M00 obey the following equation

½M00ðo; TÞ�2 þ ½M0ðo; TÞ � 1=2MsðTÞ�2 ¼ ½1=2MsðTÞ�2

ð12Þ

Thus if the only contribution to the complex dielectric

permittivity was from ionic conduction, a complex plane

plot ofM00 againstM0 would be semicircular with a radius of1=2Ms and centre on the M0 axis. If other contributions, such

as dipolar contributions were present the plot would deviate

from the semicircular shape. The modulus plots indicate

clearly more than one process occurring in the frequency

range investigated and comparison with the other data we

can ascribe the processes to local chain motion and ionic

conductivity. The modulus plots clear show the temperature

dependence of conduction process and the shift of the peak

in the dielectric modulus M00 gives an activation energy plot

shown in Figure 10. The shift in the dielectric loss e00 at

10 Hz, Figure 11, senses the higher frequency dielectric

process associated with chain motion and leads to the calcu-

lated activation energies listed in Table 3, for the homo-

polymers and their blends. A plot of the variation of the

activation energies with composition, Figure 12, shows

the variation of the activation energies with composition.

The error in determination of the activation energies was not

greater than 10%. It is evident that the energy required for

the conduction and chain reorientation processes are signi-

ficantly higher in the blends than in the homo polymers. The

increased activation energy over an above the values pre-

dicted using a simple additive relationship is indicative of

the presence of additional interactions between the chains

influencing both conduction through the matrix and the

motion of local dipoles within the matrix. These increased

activation energies are consistent with the spectroscopic

evidence of thermally generated crosslinks, hydrogen

bonding and additional interactions between neighbouring

chains. Close inspection of the dielectric modulus,

Figure 13, for PMVEMAc indicates that there are possible

Figure 10. Activation energy plots derived from the shift in thedielectric loss with temperature at 10 Hz.

Figure 11. Activation energy plots derived from shift of themodulus peak with temperature.



present three processes. The main dipole process which lies

on top of the conduction process, within that arc is the

suspicion of a second process and at high frequencies an

almost perfect semi circle indicative of a simple decoupled

process. The pure copolymer will be expected to exhibit a

variety of possible relaxations and this is demonstrated in

Figure 12. Activation Energy plots for Dielectric process in thepolymers and their blends.

Figure 13. Complex plane plots for the real and imaginary parts of the dielectric modulus for HEC,PMVEMAc and their blends over the temperature range 25–70 8C.

Table 3. Activation energy plots for the dielectric process in thepolymers and their blends.

HEC/PMVEMAc DE(e00) DE(M00)

wt.-% kJ�mol�1 kJ�mol�1

100/0 68 6480/20 79 7660/40 92 7540/60 107 8520/80 93 840/100 39 22



these plots. In contrast the HEC exhibits two processes, a

dipole process on the top of a simple conduction process.

The blends exhibit a mixture of these processes which vary

in relation to the overall composition of the blend.

Conclusion

The hydrophilic films prepared by blending of hydroxy-

ethylcellulose and maleic acid-co-methyl vinyl ether exhib-

it complete miscibility between the components, which is

due to the intermolecular hydrogen bonding. The thermal

treatment of the blends leads to polymers cross-linking via

intermacromolecular esterification and anhydride forma-

tion and the resulted materials are able to swell in water and

form hydrogels. The presence of the crosslinking reaction is

demonstrated in both infrared spectra and swelling studies

and also in TGA/DSC analysis. The thermally activated

crosslinking reactions occurring in these blends also lead to

an increase in the low temperature modulus and inhibit the

mobility of charge carriers and the motion of dipoles in the

matrix.

Acknowledgements: The assistance of Dr. David Hayward isgratefully acknowledged.

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Characterisation of Blends Based on Hydroxyethylcellulose and Maleic Acid-alt-Methyl Vinyl Ether

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