Structure-to-Properties Relationship of Aliphatic Hyperbranched Polyesters

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Structure-to-Properties Relationship ofAliphatic Hyperbranched Polyesters

Ema Zagar,* Miroslav Huskic, Majda Zigon

We investigated the effects of annealing on the rearrangement of the H-bond networkmicrostructure and its influence on the thermal and rheological properties of the secondpseudo-generation hyperbranched (HB) polyester based on 2,2-bis(methylol)propionic acid(Boltorn H20) in comparison to the fourth pseudo-generation HB polyester (Boltorn H40).During annealing the hydroxyl groups of HB polyesters form H-bonds with other hydroxyl orcarbonyl groups. The formation of –OH. . .–OH H-bonds is favored at shorter annealing times,higher annealing temperatures, and for low pseudo-generation HB polyester, which has a lowaverage molar mass, degree of branching, andcore functionality. At temperatures up to 80 8C,Boltorn H20 indicates faster and more extensiveH-bond formation as compared to Boltorn H40.This is reflected in its larger cleavage enthalpiesand higher elastic contribution to the viscoelas-tic response. At 90 8C and above, the process ofH-bond formation is delayed. Therefore, BoltornH20 exhibits only viscous behavior in rheologi-cal measurements, and in DSC curves it showsstrong exothermic transitions while heating thesample.

Introduction

Dendritic polymers, dendrimers, and hyperbranched (HB)

polymers are highly branched macromolecules with a

large number of branching points and modifiable func-

tional groups. While regular branched dendrimers have

uniform molar mass and structure, imperfect branched HB

polymers show a wide distribution in terms of molar mass,

degree of branching, spacer length between branched

points, etc.

The branched architecture of dendritic polymers plays

an important role in their behavior in solution, as well as in

E. Zagar, M. Huskic, M. ZigonNational Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana,SloveniaFax: þ386 1/476 03 00; E-mail: [email protected]

Macromol. Chem. Phys. 2007, 208, 1379–1387

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

their rheological, mechanical and processing behavior.[1]

Rheological measurements of some dendritic polymers

in solution and in the bulk state reveal much lower visco-

sities than those of their linear analogues of comparable

molar masses. Short chain branched dendritic polymers

behave like Newtonian liquids/melts with no shear thin-

ning or thickening due to lack of chain entanglements as a

consequence of surface congestion and highly branched

topology.[2] Long- and intermediate-chain branched HB

polymers with a branch length (molar mass between

branch points) much greater and comparable to the entan-

glement molar mass have much higher viscosities and

non-Newtonian flow behavior due to chain entangle-

ments.[3] The melts of HB polymers with low molar masses

and a low degree of branching (DB) could also show a

non-Newtonian behavior although their molar masses are

well below the critical entanglement molar mass. In such

DOI: 10.1002/macp.200600672 1379

E. Zagar, M. Huskic, M. Zigon

Table 1. Molar mass and structural characteristics of HB polye-sters Boltorn Hx (x¼ 20, 40)[10–12]

Parameter Boltorn H20 Boltorn H40

(core:monomer)theor. 1:12 1:60a)Mtheor:; g �molS1 1 748 7 316

b)Mw; g �molS1 1 860 6 640

b)Mn; g �molS1 920 2 580

Mw=Mn; 2.01 2.57

c)DPn; 6.75 21.37

c)Mn; g �molS1 930 2 579c)xB4 0.58 0.37c)xB2 0.42 0.63

OHether 0.06 0.004d)D 0.100 0.165d)L 0.565 0.570d)T 0.335 0.265d)DBFrey; 0.261 0.367

d)f’PP50 1.8 3.3

a)Theoretical values; Mtheor: is a dendrimer-equivalent molar mass

in which all the repeat units are attached to a core molecule; b)Mw;

Mn; Mw=Mn were determined by SEC-MALS in 0.7% LiBr/DMF;c)Structural characteristics of samples determined from their1H NMR spectra; number average degree of polymerization

(DPn;), number average molar mass (Mn;), fraction of macromol-

ecules with and without the core (xB4 , xB2 ), fraction of ether groups

(OHether);d)Structural characteristics of samples determined from

their 13C NMR spectra; contents of dendritic (D), linear (L), and

terminal (T) repeat units, degree of branching according to Frey

(DBFrey; ¼ (2D)/(2DR L)), average number of core reacted hydroxyl

groups ( f’PP50).

1380

cases, viscoelastic flow behavior of HB polymers has been

explained by partial crystallization,[4a] or H-bonds forma-

tion,[5–8] resulting in a transient quasi-network structure

that acts as an effective cross-linker. Significant inter-

molecular interactions through H-bonds were also found

to be responsible for the elastic behavior of polyamidoa-

mine dendrimers (PAMAM) in bulk at high shear rates and

frequencies.[9]

The family of HB polymers based on 2,2-bis(methylol)

propionic acid was extensively studied with respect to its

structure, reaction kinetics, molar mass characteristics as

well as solid state and solution properties.[2a,2f,4b–4n,5–14]

The rheological behavior of commercially available hydroxy-

functional aliphatic HB polyesters synthesized by a pseudo-

one-step reaction of 2,2-bis(methylol)propionic acid as the

trifunctional AB2 monomer and ethoxylated pentaery-

thritol as the tetrafunctional B4 core molecule (trade name:

Boltorn Hx; x¼ 20, 30, 40, 50, where x denotes second,

third, fourth, and fifth pseudo-generation, respectively)

has been investigated by several authors.[2a,2f,5–8] Some of

them reported that melts and solutions of Boltorn poly-

esters exhibit Newtonian behavior regardless of polyester

molar mass.[2a,2f] On the other hand, Simon et al.[7] found

that melts of low molar mass HB polyesters show shear-

thinning, whereas melts of higher molar mass polyesters

are Newtonian. Shear-thinning behavior of low molar

mass HB polyesters was explained by the stronger inter-

molecular interactions of polar hydroxyl groups. These

contradictive results could be a consequence of different

samples’ preparation procedure before measurements.

Namely, Manson et al.[6] reported that the rheological

behavior of fifth pseudo-generation Boltorn polyester

strongly depends on its thermal prehistory. Thermally

pretreated polyester at least at 160 8C prior to rheological

measurements shows Newtonian behavior, whereas the

original sample shows a viscoelastic response, which was

ascribed to the microstructure developed at storage tempe-

ratures involving H-bonding of polar groups. The effects of

H-bond network formation during annealing of fourth

pseudo-generation Boltorn H40 polyester at various

temperatures and times on its rheological and thermal

properties was studied by our group.[5] Amorphous Boltorn

H40 polyester during annealing at a given temperature

develops a more stable and extensive H-bond network

with time, which results in increasing elastic contribution

to the sample viscoelastic response and increasing enthalpy

of the endothermic transition in its DSC thermograms. The

ordering of the HB structure during annealing is a conse-

quence of formation of multiple H-bonds between long

linear sequences, which are present in HB polyester as

defects.

The primary focus of the present study was to investi-

gate the ordering of the H-bond network microstructure of

the low molar mass (second pseudo-generation) Boltorn



H20 polyester during annealing and the influence of H-

bond network microstructure on the samples’ thermal and

rheological properties. The results are compared to those

obtained for the high molar mass (fourth pseudo-gene-

ration) Boltorn H40 HB polyester.[5] The differences in

rheological and thermal properties of both samples are

related to their structural and molar mass characteris-

tics.[10–12]

Experimental Part

Materials

The HB polymers studied in this work were commercially

available hydroxy-functional aliphatic polyesters of the second

and fourth pseudo-generation synthesized from 2,2-bis(methylol)

propionic acid (bis-MPA; AB2) as the repeating unit, and ethoxy-

lated pentaerythritol (PP50; B4) as the tetrafunctional core molecule.

DOI: 10.1002/macp.200600672

Structure-to-Properties Relationship of Aliphatic Hyperbranched Polyesters

Perstorp Specialty Chemicals AB, Sweden, supplied the samples

under the trade name BoltornTMHx (x¼20, 40) polyesters, where x

denotes pseudo-generation. Our denotations of samples were H20

and H40, respectively. Molar mass and structural characteristics

of H20 and H40 were summarized in Table 1.[10–12] Before

measurements, the samples were dried in vacuum oven at 50 8Cfor 24 h.

Temperature-Dependent Infrared Spectroscopy (IR)

Infrared spectra were recorded by Perkin-Elmer PE 2000 spectro-

meter equipped with a liquid nitrogen cooled MCT detector.

Typically, 256 interferograms were collected and apodized using

the Happ-Genzel function. The spectral resolution was 4 cm�1. The

temperature measurements were performed using a diamond

Golden Gate ATR cell (Specac). The temperature controller ensured

the stabilization of the sample temperature to be within 1 8C. The

spectra were recorded between 450 cm�1 and 7 000 cm�1. The

missing part of the spectrum between 0 cm�1 and 450 cm�1 was

replaced by the theoretical descending Gaussian function. The

background spectra were collected for every recorded temperature

to eliminate the effects of the intensity disparity of the diamond

bands. Thin layer of samples were prepared from methanol

solutions, which were directly cast on a diamond ATR-crystal. The

films were treated under dry nitrogen steam in order to remove

any traces of methanol. The films were heated up to 150 8C,

subsequently cooled to 50 8C (90 8C) and tempered for 24 h. The

variations in spectra were monitored by difference spectroscopy.

The vibrations of –CH3 and –CH2 groups were used as indicators

for proper subtraction. Because these groups were unaffected by

the appearance of the H-bond structure, the bands of these

vibrations should be absent in the difference spectrum.

X-Ray Diffraction (XRD)

The samples for XRD measurements were placed in an aluminum

sample holder and heated up in an oven to 150 8C for 5 min to

remove its thermal history. The excess of the melted sample was

removed, to level the surface with the holder. To obtain quenched

samples, the holder with the sample was placed in a desiccator

for cooling to room temperature. After that, the holder with

the sample was immediately transferred to the XRD. To obtain

annealed samples the holder with the sample was transferred to

another oven heated at 50 8C. After 14 d of annealing, the sample

was placed in a desiccator, and transferred to XRD.

XRD experiments were performed using a Philips 17-10

diffractometer with Cu Ka radiation (l¼1.54 A). The scattering

intensities were detected using a scintillation counter with an

angular range 28–358 (in 2u), an angular step of 0.048, and a

measurement time of 1 s per step.

Thermal Characterization

Thermal properties of samples were investigated by differential

scanning calorimeter (DSC) Perkin-Elmer Pyris 1. In annealing

experiments, the samples were heated to 150 8C (heating rate of



10 8C �min�1) in order to remove the effect of the samples’ thermal

prehistory. They were then rapidly cooled down (cooling rate

200 8C �min�1) to the predetermined temperature (50, 60, 70, 80,

90, and 100 8C, respectively). At each temperature the samples

were annealed for different annealing times (from 1 h to 2 and 4 d

in the case of H20 and H40, respectively). After annealing, the

samples were quenched approximately 30 8C below their glass

transition temperature, i.e., H20 to �30 8C and H40 to �10 8C, for

2 min to stabilize them. The samples were subsequently reheated

to 160 8C at 10 8C �min�1. All analyses were performed under

nitrogen atmosphere.

Rheological Characterization

For rheological tests, the samples were preheated to 150 8C in order

to remove its thermal prehistory. After that, the samples were

cooled down to the predetermined annealing temperature in 2 min.

A controlled stress rheometer, Haake RS150 equipped with a

parallel plates sensor system 25 mm in diameter was used for all

rheological measurements. Depending on the annealing tempera-

ture, the gap between the plates was set to 1.2 mm. Time sweep

tests under non-destructive conditions of oscillatory shear were

performed at a constant frequency of 1 Hz. In order to perform the

measurements under linear viscoelastic response (LVR) conditions

throughout the whole experimental time of 3 h, tests were carried

out at a constant strain amplitude of 3% at 90–70 8C, and 1% of

strain amplitude at 60 8C. The upper limits of linear viscoelastic

responses of the samples were checked in preliminary measure-

ments.

Results and Discussion

Structure of Boltorn Hx polyesters

The schematic representation of H20 and H40 structures

(Scheme 1) shows the results of structural characterization

(Table 1). [10–12] Boltorn polyesters are polydisperse pro-

ducts not only with respect to their molar mass, but also to

their structure. They consist of HB structures with a polyol

core from which branches extend, and of huge amounts of

HB species without core, which are in fact tree-like bran-

ches that contain unreacted carboxyl groups mainly in

dendritic focal repeat units. HB structures without core are

produced by self-condensation of bis-MPA, which is the

main side reaction in the pseudo one-pot synthesis of

Boltorn polyesters. Since branches are of lower molar

masses, they particularly reduce the samples’ number

average molar masses, whereas the samples’ weight

average molar masses are close to their theoretical ones.

Cyclic structures formed by intramolecular esterification

reactions are not present in Boltorn polyesters, whereas a

small number of cyclic structures formed by intramole-

cular etherification reactions are present. Boltorn polye-

sters contains a high fraction of linear repeat units in the

HB structure. Therefore, their degrees of branching are low,

www.mcp-journal.de 1381


Figure 1. Difference IR spectra of H20 obtained by subtraction ofIR spectra recorded: a) At the beginning of annealing at 50 8C andat 130 8C, and b) At the end of annealing after 24 h at 50 8C and at130 8C.

Scheme 1. Schematic structure of H20 and H40 HB polyesters.

1382

well below the theoretical value of 0.5. Low DBs are

consequence of limited carboxyl group conversion, a low

degrees of polymerization (DPn;), the presence of the core

unit, and most probably also due to the lower reactivity of

hydroxyl groups in linear repeat units compared to those

in terminal ones. The DBFrey; , according to Frey,[15] which is

considered to be more accurate for low molar mass HB

polymers,[15–18] is higher for H40 (0.37) than H20 (0.26).

According to Frechet[19], the DBFrechet are higher and do not

change substantially with pseudo-generation (0.430 for

H40 and 0.435 for H20) since the branching perfection

according to this definition is overestimated at low

DPn.[15–18] The difference between both samples is also

in the average number of reacted hydroxyl groups of the

core, i.e., average core functionality ( f’PP50), which is 1.8 for

H20 and 3.3 for H40.

To summarize, the major differences between H20 and

H40 HB polyesters are in average molar mass, average

degree of branching, content of HB structures without the

core molecule, and average number of reacted hydroxyl

groups of the core (Table 1, Scheme 1).

The results of previous studies[5–8,10–14] of Boltorn

polyesters reveal that a large number of polar hydroxyl

groups in HB polyesters form H-bonds with proton

acceptor groups, e.g., another hydroxyl, carbonyl, and

carboxyl groups, at room temperature. H-bonds weakened

with rising temperature and cannot be completely dis-

rupted only by dissolving the samples in polar organic

solvents. During annealing of fourth pseudo-generation

H40 at temperature above its glass transition, the H-bond

network microstructures rearrange towards a more or-

dered structure, which significantly influences its thermal

and rheological properties. The comparison of results for

H40 to those obtained for its fractions and to an ideally

branched dendrimer analogue revealed that structure

ordering is a consequence to the formation of multiple

H-bonds between long linear sequences, which are present

in the HB structure as defects.



According to the above-mentioned structural and molar

mass differences between H20 and H40, we would expect

H20 to have a different degree of structural ordering under

the same annealing conditions and that there would,

consequently, be differences in thermal and rheological

properties for both samples.

IR Spectroscopy

The structural changes in H20 were tracked by difference

IR spectra recorded at the beginning of the annealing

process at 50 8C and 130 8C (Figure 1a) and at the end of the

annealing process after 24 h at 50 8C and 130 8C (Figure 1b).

H20 difference spectra are similar to the difference spectra

of H40 (Figure 2), where the number of proton donors

and acceptors involved in H-bond formation is higher

at 50 8C than at 90 8C. Besides, at 90 8C a relatively

higher fraction of –OH. . .–OH H-bonds are formed com-

pared to –OH. . .O––C<H-bonds, indicating that H-bonds

between hydroxyl groups are thermally more stable.

DOI: 10.1002/macp.200600672


Figure 2. Difference IR spectra of H40 obtained by subtraction ofIR spectra recorded at the end and at the beginning of the 24 hannealing period at temperatures: A: 90 8C; B: 50 8C.

The difference IR spectra of H20 also show the donor or

acceptor groups of hydrogen atoms indicating that the

H-bond structure of both samples rearranges in a similar

manner. The main distinction between both samples is

that H-bonds, particularly those between hydroxyl groups,

are in the case of H20 already formed during the cooling

down from 130 to 50 8C (Figure 1a). During annealing

of H20 the relative amount of –OH. . .O––C<H-bonds

increases (Figure 1b). After 24 h of annealing at 50 8Cthe relative fraction of –OH. . .–OH as compared to that

of –OH. . .O––C<H-bonds is higher in H20 than in H40 as

revealed by a comparison of the difference spectra for both

samples (Figure 1b and Figure 2b).
Figure 3. a. X-ray diffractograms of quenched Boltorn H20 andH40; b. X-ray diffractograms of annealed (50 8C, 14 d) Boltorn H20and H40.
X-Ray Diffraction Measurements (XRD)

The XRD diffractograms of quenched H20 shows a some-

what sharper amorphous halo than H40 indicating some

ordering of the structure during the X-ray measurement

performed at room temperature (Figure 3a).

After annealing, the position of the peak maximum in

X-ray diffractograms for both samples remains unchanged

(Figure 3b). However, the peak maxima at 2u¼ 17.628 for

H20, and 17.368 for H40 indicates that the inter-chain

distance in H20 (0.509 nm) is somewhat smaller than in

H40 (0.516 nm) meaning a slightly denser chain packing

in H20. This finding supports the IR results, i.e., a higher

content of –OH. . .–OH H-bonds in comparison to –OH. . .

O––C<H-bonds for H20 than for H40. The shape of the

amorphous halos after annealing becomes much sharper,

indicating ordering of the HB structure of Boltorn poly-

esters through H-bond interactions. The peaks that would

indicate the presence of a crystalline structure in Boltorn

polyesters were not present in their diffractograms.



Rheological Characterization

The formation of H-bond network microstructure during

annealing of the Boltorn H20 at various temperatures

(60–90 8C) was tracked by rheology measurements follow-

ing the complex viscosity (h�) and phase lag (d) over time

(Figure 4). Over the whole time range (3 h), the complex

viscosity of H20 monotonically decreases with tempera-

ture. At the lowest temperature (60 8C), the sample exhibits

a pronounced increase in viscosity coinciding with the

appearance of the elastic contribution (d below 908)already at the beginning of annealing process, which

means that H-bonds were formed to some extent during

the cooling of the sample from 150 8C to 60 8C. With

increasing temperature, the induction time for H-bond

network formation gets longer. This was reflected by the

delayed start of a pronounced increase in viscosity and

decrease in phase lag. Due to extensive H-bond formation,



Figure 4. Complex viscosity (h�) and phase lag (d) as a function oftime under nondestructive conditions of oscillatory tests atfrequency of 6.28 rad � s�1 under the conditions of LVR for BoltornH20 at 60, 70, 80, and 90 8C.

1384

the phase lag after the induction time decreases much

faster than the increment of melt viscosity, meaning that

H20 shows higher elastic than viscous contribution to its

viscoelastic response. In contrast, at the highest tempera-

ture (90 8C), H20 shows only viscous behavior (phase

lag¼ 908), thus indicating the absence of intermolecular

interactions through H-bonds within the 3 h experimental

time.

A comparison of the obtained rheological results of H20

to those of H40,[5] (an example is given in Figure 5 for an

annealing temperature of 70 8C) reveals that at all anne-

aling temperatures, H20 shows lower complex viscosities

than H40. At temperatures between 60 8C and 80 8C, H20

exhibits shorter induction time to begin the structural

buildup by H-bond formation, while the elastic contribu-

tion to its viscoelastic response is higher than in the case of

Figure 5. Complex viscosity (h�) and phase lag (d) as a function oftime under nondestructive conditions of oscillatory tests atfrequency of 6.28 rad � s�1 under the conditions of LVR for BoltornH20 and H40 at 70 8C.



H40. In contrast to H40, at 90 8C H20 shows only viscous

behavior within the 3 h experimental time.

Thermal Characterization

The formation of H-bond microstructure during annealing

at various temperatures and times was also tracked via

DSC measurements by monitoring the H-bond cleavage

enthalpy from the heating scans of the isothermally

annealed samples (for sample preparation see experi-

mental part). The broad endothermic transition in DSC

thermograms of H20 shifts toward higher temperature

with increasing annealing temperature and time as a

consequence of the formation of a thermally more stable

H-bond microstructure (Figure 6a, 6b, 9a). The endothermic

transition is composed of two poorly resolved peaks whose

position depends on annealing temperature and time.

The low-temperature peak is more intensive at annealing

temperature below 70 8C, whereas above 70 8C the

Figure 6. a. DSC curves of Boltorn H20 annealed at differenttemperatures for 4 h; b. DSC curves of Boltorn H20 annealedat 70 8C for various annealing times.

DOI: 10.1002/macp.200600672


Table 2. Kinetic data (DH0, k, Equation (1)) for the H-bond clea-vage process in Boltorn H20 and H40. (Missing values are due tothe presence of exothermic transitions at around 40 8C in DSCcurves of annealed H20, which prevent accurate DH determi-nation)

Temp. H20 H40[5]

DH0, K DH0, k

-C J � gS1 J � gS1

50 15.2 1.88 12.3 1.92

60 11.8 2.30 10.6 1.87

70 9.2 2.67 8.9 1.74

high-temperature peak prevails (Figure 6a). On the other

hand, at 70 8C, the low-temperature peak prevails at

shorter annealing times and the high-temperature peak at

longer annealing times (Figure 6b). Consequently, the peak

temperature shifts to higher values according to the

prevailing temperature peak; from around 25 8C above

annealing temperatures at 50 and 60 8C, to around 37 8Cabove annealing temperature at 70–90 8C (Figure 9a).

The number of H-bonds formed during annealing can be

correlated with the enthalpy (DH) of the endothermic

transition in the DSC curves. DH increases with decreasing

temperature and increasing annealing time. In the latter

case the dependence of DH on the logarithm of time

(Equation (1)) is linear. (Figure 7, Table 2).

Fig(DHtem

80 2.4 4.14 6.2 1.44

90 — — S1.6a) 4.10a)

Macrom

� 2007

DH ¼ DH0 þ k lnðtÞ (1)

100 — — S5.2 3.00

a)Extrapolated values.

where DH is the enthalpy of the H-bond cleavage, DH0 is

the enthalpy of H-bond cleavage at t¼ 1 h, k is the slope of

the straight line representing the rate of H-bond cleavage,

and t is the annealing time (h).

The formation of H-bonding microstructure during

annealing has negligible effect on the glass transition

temperature (Tg), but a larger one on the heat capacity (Dcp)

at glass transition, which decreases with increasing entha-

lpy (Figure 8) as a consequence of restricted molecular

mobility.

The obtained results for the thermal characterization of

H20 show that annealing affects the thermal properties

similarly to H40. However, a detailed comparison of ther-

mal properties of both samples points out some significant

differences. The enthalpy of H-bond cleavage at all anne-

aling times between 50 and 70 8C as well as the rate of

H-bond cleavage (k) is higher for H20 than H40 (Table 2). At

80 8C, the k value is higher in the case of H20 than H40,

ure 7. The dependence of hydrogen bond cleavage enthalpy) of Boltorn H20 as a function of annealing time at variousperatures.

ol. Chem. Phys. 2007, 208, 1379–1387

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

whereas the DH values of H20 become larger than those of

H40 at somewhat longer annealing times. In contrast to

H40, H20 shows strong exothermic transitions at tem-

perature of 90 8C and above, signifying that majority of

H-bonds are formed during sample heating. An endother-

mic transition of low intensity appears only at annealing

time longer than 4 h, which is consistent with the rheo-

logical results of H20. Since exothermic and endothermic

peaks are not well resolved, DH values of H20 cannot be

accurately determined at �90 8C. Furthermore, the tem-

perature at which the high-temperature peak becomes

more intensive than the one at low-temperature is lower

in the case of H20 (70 8C) than H40 (90 8C) (Figure 6, 9).

However, the difference between Tg and the temperature

at which the high-temperature peak prevails over the low

Figure 8. Dependence of heat capacity (Dcp) at glass transition onhydrogen bond cleavage enthalpy (DH) for Boltorn H20 and H40.



1386

temperature peak is larger for H20 (68 8C) than for H40

(59 8C).

The presented results reveal that during annealing

between 50 and 80 8C, H20 develops faster H-bond

microstructure and to a higher extent than H40, despite

the difference between annealing and glass transition

temperatures for H20 is larger than that for H40 (Tg,H20 ¼2 8C, Tg,H40 ¼ 31 8C). In contrast, when the annealing

temperature is 90 8C or more, the rate of H-bond formation

is slower for H20 than for H40. Above this critical tempera-

ture a high mobility of H20 molecules restricts H-bond

formation so that only more stable H-bonds are formed.

The differences in the obtained results can be explained

by the molar mass and structural characteristics of both

samples (Table 1, Scheme 1). H20 has a low average molar

mass and hence a low melt viscosity and high overall

mobility of molecules. It has an open structure with a very

high content of hydroxyl functional groups in linear and

terminal repeat units, and a very low content of fully

Figure 9. a. Peak temperature as a function of time for BoltornH20 annealed at temperatures from 50 to 90 8C; b. Peak tempera-ture as a function of time for Boltorn H40 annealed at tempera-tures from 50 to 100 8C.



reacted dendritic repeat units. Such structure enables the

formation of multiple H-bonds between the long linear

sequences in the HB structure, which results in fast and

efficient structure ordering below 90 8C. Besides, linear

sequences are easily accessible due to low average core

functionality and high content of low molar mass HB

structures without the core molecule.

In contrast to H20, H40 has a higher average molar mass,

higher average core functionality and viscosity, a more

branched structure with a higher content of dendritic

repeat units, a lower content of terminal and a comparable

content of linear repeat units. For these reasons the

interpenetration of branches and consequently the for-

mation of multiple H-bonds are more restricted in the case

of H40. In addition, because the interior structure of

H40 has higher content of dendritic repeat units than

H20, the relative fraction of –OH. . .O––C<H-bonds com-

pared to –OH. . .–OH H-bonds is higher in H40 than in H20.

In conclusion, the low molar mass H20 shows stronger

H-bond interactions in the melt at annealing temperatures

between 50–80 8C than high molar mass H40 not only due

to a higher fraction of hydroxyl groups in terminal repeat

units as was explained by Simon et al.,[7] but also due to

the higher mobility and easier accessibility of hydroxyl

groups for H-bonding in the linear repeat units. The order-

ing of HB structure in Boltorn polyesters is more likely

a consequence of interpenetration of branches through

formation of multiple H-bonds between linear sequences

than back folding of linear sequences as was explained by

Hiltner et al.[14]

Conclusion

During annealing of Boltorn HB polyesters, their structure

become more ordered as a consequence of the formation of

intermolecular multiple H-bonds between long linear

sequences rather than back folding of branches. The rate of

H-bond formation and the kind of H-bond microstructure

formed depend not only on annealing temperature and

time but also on molar masses and structural character-

istics of Boltorn HB polyesters (average molar mass, bran-

ched structure, and average number of reacted hydroxyl

groups of the core). At temperature between 50 and 80 8Cthe formation of H-bonds is faster and more extensive for

the second pseudo-generation HB polyester H20 compared

to the fourth pseudo-generation HB polyester H40. This is

due to the higher mobility of low molar mass macro-

molecules as well as easily accessible hydroxyl groups

for H-bond formation not only in the terminal but also

in linear repeat units. Therefore, the relative fraction

of –OH. . .–OH H-bonds compared to –OH. . .O––

C<H-bonds is higher in H20 than in H40. The formation

of –OH. . .–OH H-bonds is also favored at shorter annealing

DOI: 10.1002/macp.200600672


time and higher temperature, indicating that –OH. . .–OH

H-bonds are thermally more stable than –OH. . .O––

C<H-bonds. At 90 8C and above, H20 is well above its

glass transition temperature and its induction time for

H-bond formation is longer than in the case of H40 since

high mobility of molecules restricts formation of H-bonds.

Since the rate and type of H-bond microstructure for-

mation depend on molar masses and on the structural

characteristics of Boltorn HB polyesters, the annealing of

H20 and H40 affects their rheological and thermal pro-

perties differently. At temperatures up to 80 8C, H20 shows

shorter induction time for a pronounced increase in the

complex viscosity and a decrease in the phase lag, which

indicate the beginning of the H-bond formation process.

Compared to H40, H20 also shows a higher elastic contri-

bution to its viscoelastic response. These results are consis-

tent with higher values of cleavage enthalpies and rates of

H-bond cleavage of H20 as compared to the corresponding

values for H40. At 90 8C and above the process of H-bond

formation for H20 is due to high mobility delayed, which is

reflected in the viscous behavior only and in the presence

of strong exothermic transitions in DSC curves indicating

the formation of H-bonds during heating the sample.

Acknowledgements: The authors gratefully acknowledge thefinancial support of the Ministry of Education, Science and Sportof the Republic Slovenia (program P2-145). The authors are alsograteful to Dr. Andreja Zupancic-Valant, Faculty of Chemistry andChemical Technology, University of Ljubljana, Slovenia, and Dr.Joze Grdadolnik, National Institute of Chemistry, Ljubljana,Slovenia, for carrying out rheological and IR spectroscopicmeasurements, respectively.

Received: December 28, 2006; Revised: March 2, 2007; Accepted:March 9, 2007; DOI: 10.1002/macp.200600672

Keywords: hyperbranched; polyesters; structure-property rela-tions; thermal properties; viscoelastic properties

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Structure-to-Properties Relationship of Aliphatic Hyperbranched Polyesters

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