Effect of Water on the Glass Transition Behavior of Hydrophilic

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch012

12 Effect of Water on the Glass Transition Behavior of Hydrophilic Polyurethanes with Mixed Soft Segment Nathaniel S. Schneider, David A. Langlois, 1

2,3

and Cathyrine A. Byrne

2

Geo-Centers, Inc., 7 Wells Avenue, Newton, MA 02159 Polymer Research Branch, AMSRL-MA-PB, Army Research Laboratory, Materials Directorate, Watertown, MA 02172 1

2

Water uptake and glass transition behavior in a series of hydro philic polyurethane elastomers with a soft segment of poly(eth ylene oxide) (PEO) alone or in a mixture with poly(tetramethylene oxide) (PTMO) were studied. In a set of samples with various PEO/PTMO ratios at fixed hard-segment content, the saturation water uptake was almost constant at 2.5 mol of water per mol of ethylene oxide. Differential scanning calorimetry measurements showed two separate glass transition temperatures (T ) in the dry, mixed-soft-segment samples (-50 °C for PEO and -80 °C for PTMO) but only a single T at about -80 °C in the wet samples. The ΔC values for the wet samples were much higher than those for the dry samples. Measure ments of the sample with pure PEO from the set just described indicated an essentially linear dependence of both T and Δ C on water content. g

g

p

g

p

H Y D R O P H I L I C P O L Y ( E T H Y L E N E O X I D E ) (PEO) containing polyure thanes that are based on the use of a mixed soft segment to control the extent of water swelling were first described by Tobolsky and co3

Current address: Los Alamos National Laboratory, MS E549, Los Alamos, NM 87545 0065-2393/96/0248-0195$12.00/0 © 1996 American Chemical Society

In Hydrophilic Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


196

HYDROPHILIC POLYMERS

workers (1 ). Their study included measurements of water uptake and water and salt permeabilities and emphasized the relation of these processes to possible reverse osmosis applications. The variable and controlled degree of swelling with the related water and solute permeability is also of interest in biomedical applications of polymers as wound coverings and for controlled release. Additionally, these materials might be useful for moisture-permeable coated fabrics. Tobolsky and co-workers showed that the saturation water content was directly proportional to the P E O content in the mixed PEO-poly(propylene oxide) soft segment; obviously, then, little interaction occurs between these two components. Hunger et al. (2, 3) followed an alternative approach to hydrophilic polyurethanes based on the use of a block copolymer polyether consisting of a central segment of poly(propylene oxide) and terminal segments of P E O . These samples showed more complex dependence of water uptake on the soft-segment composition and the temperature. A recent reexamination of the data (4) suggested that the complex behavior is due to the incompatibility of the two components of the soft segment. The present study is concerned with the water uptake and glass transition behavior of a series of hydrophilic polyurethanes prepared with a mixed soft segment of P E O and poly(tetramethylene oxide) (PTMO).

Experimental Details Polyurethane samples were formed from methylene bis(4-cyclohexylisocyanate) (H12MDI), butanediol (BD), and either a single polyol or a mixture in several mole ratios. P E O (molecular weight 1450) (Union Carbide) and P T M O (nominal molecular weight 2000) were used for the soft segment. The polyurethanes were formed by the two-stage reaction consisting of end capping with H12MDI, using dibutyl tin dilaurate as catalyst, followed by chain extension with BD. The cure was completed in a closed mold divided into equal 20- and 50-ml sections that was heated in an oven at 100 °C for 16 h. Equilibrium sorption measurements were performed on preweighed 1-in (2.54-cm)-diameter 50-ml discs that were immersed in distilled water maintained at 30 °C. Differential scanning calorimetry (DSC) runs were made using a Perkin-Elmer DSC-2 with a liquid nitrogen reservior and helium purge gas. Samples cut from the 20-ml section of the molded sheet, weighing approximately 15 mg each, were initially quench cooled and then heated from -120 to 40 °C at 20 °C/min. A repeat run, following cooling at 40 °C/min, exhibited reproducibility within ± 1.5 °C for T and ± 5 % for AC . For runs on the wet samples, the weight of the watersaturated discs was determined and the dry sample weight was calculated from the previously determined equilibrium water uptake. A different procedure was used for examining the effect of various water contents on the T behavior of 4PU10 (see "Results" for a description of sample designations). Discs of known dry weight were first saturated with water and then allowed to come to the desired final water concentration by s

p

g


12.

SCHNEIDER E T AL.

Effect of Water on Glass Transition Behavior

197

evaporation of the excess water before they were sealed into crimped aluminum pans. The D S C scans were made at least 3 h following encapsulation to allow for uniform distribution of water throughout the sample thickness. A check one or more days later indicated that no water loss had occurred and that the D S C behavior reproduced the original result.


Results The composition and water uptake of the set of hydrophilic polyurethanes are presented in Table I. The sample designations used in that table indicate sample compositions. The first number is the number of moles of diisocyanate used for 1 mol of total polyol. Since all the compositions were stoichiometric, this number completely specifies the molar composition. The number following P U indicates the weight percent of P E O in the mixed soft segment. Thus 4PU6 designates a sample with 4 mol of diisocyanate, 3 mol of butanediol, and 1 mol of polyol consisting of 60 wt% P E O and 40 wt% P T M O (0.678 mol of PEO/0.322 mol of P T M O ) . The water uptake, recorded in Table I as grams of water per 100 g of polymer, increases progressively with increasing P E O in the first four samples, from 4PU5 to 4PU10. The next column of data, which presents water as moles of water per mole of ethylene oxide (EO), indicates that the water uptake is essentially proportional to the P E O content in these four samples of fixed hard-segment content and increases only slightly with increasing P E O content. With decreasing hard-segment content (samples 4PU10 to 2PU10), water uptake increases markedly. This increase indicates the decreasing resistance to swelling imposed by the hard-segment structure, which functions as a physical cross-link network. In the dry samples with mixed soft segment, the glass transitions of the two individual polyethers are clearly discernible, as illustrated Table I. Water Uptake in Hydrophilic Polyurethanes Water Uptake

Sample

PTMO (gig of polymer)

PEO (gig of polymer)

%

Mol H2O/M0I EO

4PU0 4PU5 4PU6 4PU7 4PU10 3PU10 2PU10 2.7PU5

0.606 0.285 0.221 0.163 0.000 0.000 0.000 0.330

0.000 0.279 0.332 0.382 0.523 0.599 0.702 0.327

1 26 33 39 57 80 145 39

2.28 2.43 2.50 2.67 3.26 5.05 2.92


198



5.0

-100.0

-75.0

-50.0

-25.0

0.0

25.0

TEMPERATURE (° C) Figure 1. DSC scan of 4PU5 in the dry condition. Sample weight, 14.8 mg; scan rate, 20 °C/minfor -120 to 30 °C. by the example i n F i g u r e 1. T h e corresponding T s , w h i c h w e r e determ i n e d as the m i d p o i n t of the increase i n heat capacity, are recorded i n T a b l e II. T h e T of - 7 8 °C for 4 P U 0 is typical of values reported for P T M O i n M D I - b a s e d polyurethanes a n d is close to the value of — 85 °C for the pure soft segment ( M D I is d i p h e n y l m e t h y l d i i s o c y a nate) (5). H o w e v e r , uncertainty about the T of P E O is considerable, g

g

g

Table II. Soft-Segment Thermal Transition Behavior âC [JKgofSS-K)]

ÂC [JI(gofPEO-K)]

Wet

Dry

Wet

Dry

Wet

-79 -75 -75 -78 -79

0.50 0.62 0.71 0.76 0.89 1.03 1.05° 0.79

0.55 1.20 1.39 1.44 1.43

0.74 0.86 0.87 0.89 1.03 1.05 1.09

1.92 1.99 1.84 1.43

p

τ Sample

Dry

4PU0 4PU5 4PU6 4PU7 4PU10 3PU10 2PU10 2.7PU6

-78 -82, -83, -82, -51 -51 -52 -81,

β

-52 -52 -50

cc)

n.d.

-52

-66 -78

n.d.

1.72 1.40

P

NOTE: SS is soft segment; n.d. is not determined. Corrected for 15% crystallinity.

a


n.d.

1.72 2.31

12.


SCHNEIDER E T AL.

199

presumably because of the effect of the high degree of crystallinity of the pure polyether. The value of - 50 °C observed here is probably elevated somewhat above the value for the pure polyether by hydrogen bonding to phase-mixed urethane segments. In the wet samples the two glass transitions coalesce, as shown by the example in Figure 2, and only a single T is observed at - 75 °C or lower, as recorded in Table II. In addition, the water melting peak is sharp and increases in size through the set of samples with increasing P E O in the mixedsoft-segment samples. This peak represents less than 1% of the water present in these samples. Thus essentially all of the water is present as nonfreezing bound water. The samples with a pure P E O soft segment exhibit more complicated behavior in the wet state, which, although not illustrated by additional figures, is described in what follows. In 3PU10 the T is obscured by soft-segment recrystallization, and additional endothermal activity occurs below and above the water melting peak. The lower endothermal region can be attributed to bound freezing water and amounts to about 4.5% of the added water. This amount is about 4 times more water than appears in the sharp melting peak at 0 °C. A peak at 19 °C represents the melting of water-induced crystallinity in the P E O segment. These complications make it difficult to quantify


g

g

6.0

-100.0

-75.0

-50.0

-25.0

0.0

25.0

TEMPERATURE (° C) Figure 2. DSC scan of 4PU5 with saturation water uptake. Sample weight, including dissolved water, 15.9 mg; scan rate, 20 °Clmin for -120 to 30 °C.


200


the behavior at T in this sample. 2PU10 shows 15% crystallinity in the dry sample, but in the wet sample the melting peak at 18 °C is erased, indicating that P E O is soluble in the higher-saturation water content of this sample. A large water peak, amounting to less than 5% of the added water, is centered at - 2 °C. Otherwise, the region be tween T and the melting peak of water is relatively clear, and, there fore, the T behavior can be analyzed. Some further indication of the effect of water on soft-segment be havior can be gained by a quantitative examination of the heat capacity change at the soft-segment glass transition. The results are recorded in Table II in joules per gram of soft segment per degree Kelvin. For the mixed-soft-segment samples, the recorded AC s were based on the temperature range, which included the transitions due to both types of polyol (see Figure 1). In comparing the A C s for these samples in the dry and wet states, significantly higher values in the wet state are immediately evident. A n additional important observation is that AC ρ increases progressively with increasing proportion of P E O in the soft segment through the set of samples from 4PU0 to 4PU10. This indicates that AC is larger for P E O than for P T M O . In 4PU10 the estimated A C in the wet state is lower than that of the mixed-softsegment samples. This result might have been affected by the broad low-temperature endotherm, which probably represents bound freez ing water. As noted in the previous paragraph, interfering thermal processes also complicated the interpretation of the T behavior in the other wet samples with pure P E O soft segment. In the phase-segregated polyether polyurethanes, separation of soft- and hard-segment components is usually incomplete. Short hardsegment units can mix with the soft-segment phase and raise the T . In addition, a portion of the soft segment can mix with the hard-seg ment phase or might be immobilized at the interphase and will not contribute to the behavior at T . One possible explanation of the in crease in A C is that a change in morphology has occurred with the addition of water and that the fraction of free soft segment has accord ingly increased. A n estimate of the fraction of free soft segment, F , is given by the ratio (5) g

g


g

p

p

P

p

g

g

g

p

s s

Fss -

à C

o

W

where A C is the measured value, and AC ° is the value for the pure soft segment. The value of AC ° for pure P T M O is 0.815 J/(gK) (5). The fraction of free soft segment for P T M O , determined as the ratio of the measured value (Table II) and the value for the pure sample, p

P

P


12.

SCHNEIDER E T AL.


201

is 0.61, which is close to the value reported for P T M O in an M D I - B D polyurethane. As noted earlier in this chapter, there is no reliable A C ° for P E O because it is not possible to measure AC direcdy due to the high level of crystallinity in P E O . A value for AC ° can be estimated from the set of samples in the dry state with fixed hard-segment contents, that is, 4PU0 to 4PU10, by subtracting the P T M O contribution. The P T M O contribution in the mixed-soft-segment samples, A C ( P T M O ) , is given by the following relation: p

P

P


p

4 C ( P T M O ) = wF AC ° p

ss

(2)

p

where all quantities on the right-hand side refer to P T M O , and w is the weight fraction of P T M O in the soft segment (Table I). The resulting AC s for P E O for the three samples 4PU6 to 4PU10 (next-to-last column in Table II) are almost constant and equal 0.87 J/(g-K). Thus, the results support the assumptions underlying the calculation. Why the value for 4PU5 is distinctly lower is not known. If a A C ° for P E O is estimated by assuming that F = 0.61 (the same as that for PTMO), the resulting value is 1.43 J/(g-K), which is far larger than that of P T M O . However, even this value is below the AC results of nearly 2 J/(g-K) for the wet samples (last column of Table II). Furthermore, given the nonpolar nature of H12MDI compared to M D I , it is likely that F is greater for P E O than for P T M O , with the result that AC ° for P E O would be lower than the value just indicated. Thus the higher values for the wet samples cannot be attributed to an increase of the free P E O to 100% as a result of water uptake. In fact, the comparison suggests that water contributes direcdy to AC . To determine the contribution of water to T behavior, D S C measurements were conducted with another preparation of 4PU10 at four water contents from 15.8 to 50% (0.302 to 0.96 g of H 0 / g of P E O ) ; the highest value corresponded to the saturation concentration in this sample. This sample differed from that on which the earlier measurements of Tables I and II were made, and some of the properties differed from those of the earlier sample. For all water concentrations below saturation, the D S C traces were free of complicating features that would interfere with a reliable measurement of AC . However, at saturation, the D S C scan was complicated by a broad peak at about — 25 °C, which represented bound freezing water, and a moderately sharp peak at about 2.5 °C. These features precluded the determination of an accurate value of A C in this sample, although it was still possible to determine T . The T results, plotted as a function of water concentration in Figure 3, follow a surprisingly linear dependence, with a slope of - 33.5 K/g of H 0 . A C also exhibits a clear-cut dependence p

p

s s

P

s s

P

P

g

2

P

p

g

g

2

p


202



225

190

"i

0

1

1

·

0.2

1 0.4

1

1 0.6

1

1

1

•

0.8

1

g Water/g PEO Figure 3. Plot of polymer T for 4PU10 as a function of water uptake in units of grams of H2O per gram of PEO. g

0.75 H 0

«

1 1 1 1 1 1 1 0.2 0.4 0.6 0.8

«

g Water/g PEO

1 1

Figure 4. Plot of àC for 4PU10 as a function of water uptake in units of grams of H2O per gram of PEO. Error bars represent standard deviations for repeat scans on a single sample. p


12.

S C H N E I D E R E T AL.


203

on water content. The data are plotted in Figure 4, where the vertical segments indicate the standard deviation from four or more repeat runs. The dependence on water content is almost linear, and the slope of the line leads to AC = 1.45 J/(g of H O K ) . 2

P


Conclusions In the set of samples with fixed hard-segment content (4PU6 to 4PU10), the water uptake in the samples with mixed soft segment is approximately proportional to the E O content, that is, a ratio of 2.5 mol of H 0 / m o l of E O . The D S C trace shows that essentially all the water is bound nonfreezing water. If all the P E O in the soft segment is assumed to be accessible to water, then this value should represent the water-EO stoehiometry. Although this value is in conflict with the results of deuterium N M R data on water in a highly crystalline P E O sample (6), which indicated a one-to-one stoichiometry, it is in agreement with the results of chemical shift N M R results (7), which suggested a molar stoichiometry of 3 H 0 / E O . The ratio is even higher in the samples of lower hard-segment content, but the increasing com plexity of the D S C trace makes it difficult to determine quantitatively the fraction of added water that is not freezing. Water has a profound effect on the glass transition behavior, in cluding a depression of the soft-segment T and an increase in AC . The T depression exhibits a linear dependence on water content. The increase in A C also appears to be proportional to water content. The nature of the underlying physical process responsible for the increase in A C is not known but is probably related to the interactions with P E O represented by the bound nonfreezing state of all or a large frac tion of the water content. One possible interpretation of these results is that a loss of water mobility accompanies the freezing out of the polymer segmental mobility at T . Deuterium N M R experiments could be helpful in providing a direct determination of the state of water and the changes in water mobility that might be occurring at the polymer T . 2

2

s

P

g

p

p

g

g

References 1. Chen, C. T.; Eaton, R. F.; Chang, Y. S.; Tobolsky, Α. V. J. Appl. Polym. Sci. 1962, 16, 2195. 2. Illinger, J. L.; Schneider, N. S.; Karasz, F. E. In Permeability of Plastic Films and Coatings to Gases, Vapors and Liquids; Hopfenberg, H. D.; Ed.; Plenum: New York, 1975; pp 183-196. 3. Illinger, J. L. In Polymer Alloys; Klempner, D.; Frisch, K. C., Eds.; Plenum: New York, 1977; pp 313-325.


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POLYMERS

4. Schneider, N. S.; Illinger, J. L.; Karasz, F. E. J. Appl. Polym. Sci. 1993, 47, 1419.

5. Camberlin, Y.; Pascault, J. P. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 1835.

6. Hey, M. J.; Ilett, S. M . ; Mortimer, M.; Oates, G. J. Chem. Soc., Faraday Trans. 1990, 86, 2673.

7. Liu, K. J.; Parsons, J. L. Macromolecules for review November

18,1993. A C C E P T E D

revised manuscript July


RECEIVED 26, 1994.

1969, 2, 529.


Effect of Water on the Glass Transition Behavior of Hydrophilic

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