Macromolecules 1994,27, 4919-4926
4919
Phase Structures of Hexamine Cross-Linked Novolac Blends. 1. Blends with Poly(methy1methacrylate) Xiaoqing Zhang and David H. Solomon' School of Chemistry, The University of Melbourne, Parkville, VIC 3052, Australia
Received February 15, 1994; Revised Manuscript Received May 23, 1994.
ABSTRACT The miscibility and phase structures of initially miscible novolac/poly(methylmethacrylate) (PMMA)blends after full curing of the novolacare investigated by high-resolution solid-stateNMR, FT-IR, and DSC techniques. The strong intermolecular hydrogen-bondinginteraction between novolac and PMMA results in the blends being miscible at the molecular level, and the phase-separation process of the blends is very slow even at 200 O C . Curingnovolac/PMMAblends with hexaminereducesthe intermolecularhydrogen bonding significantly, but a considerable amount of residual intermolecular hydrogen bonds still remains in the cured blends. Thephase structures of the fullycurednovolac/PMMAblends showcompositiondependence. Fully cured novolac-richblends result in semi-interpenetratingnetworks;novolac forms a highly cross-linked network through the whole blend while PMMA chains distribute uniformly in the network with domain sizes on 2-3 nm scales. The segmental motion of the PMMA chains is frozen by the network, and the glass transition is not observed. When PMMA is rich, cross-linkednovolac chains distributein the PMMA matrix. Most fully cured blends are partially miscible or microphase-separated systems with domain sizes around a scale of 20-30 nm. The phase structure and properties of phenolic resins can be modified and extended by blending novolacwith a high polymer with careful choiceof the blendingcompositionand curing conditions.
Introduction Phenol-formaldehyde resins have been used commercially as molding compounds, laminates, adhesives, and shell molds for metals and electrical insulation, because of their low manufacturing cost, dimensional stability, age resistance, and high tensile strength. Blending novolacs with other polymers is a simple, economic, and effective method to modify and extend the properties of phenolic resins. As described in previous reports,l4 with intermolecular hydrogen bonding as a dominant driving force, novolac resins were found to be miscible with polymers which contain carbonyl or carbonate groups. The low molecular weight of novolacs, usually in the range 10002000, is also beneficial for novolacs to form a miscible blend with a high polymer. Most novolac resins are utilized in a cured form. Chemicalcuring results in chain extension, branching, and cross-linking. If these occur to an initially misciblenovolac blend, the hypothetical lower critical solution temperature (LCST) will fall, and phase separation may occur under conditions of sufficient mobility of polymer chains and favorable kinetics. If curing is carried out for arigid blend a t a relatively lower temperature where polymer chains are not mobile and the rate of phase separation is very slow, a semi-interpenetrating network (semi-IPN) could form after curing, and the system may still be miscible or partially miscible. The strength of intermolecular interaction, the composition of the blend, and the cross-link density should also play key roles for phase structures of novolac blends after curing the novolac component. Novolac/poly(methyl methacrylate) (PMMA) blends were chosen for the initial study. Other systems will be reported later. The miscibility in IPNs has been addressed by theoretical treatment for networks with low cross-link density and weak interactions.6 Some reports have also described the cross-linking effect on the miscibility of novolac-rich blends (80% of novolac in blends) with intermolecular hydrogen bonding as a dominant driving force for the mi~cibility.~?~ After curing the novolac
* To whom correspondence should be addressed. @
Abstract published in Adoance ACS Abstracts, July 1, 1994. 0024-929719412227-4919$04.50/0
component,the amount of intermolecular hydrogen bonds decreased, and the bonding only reestablished slightly during a cooling cycle. The cross-link density of novolac in their samples was relatively low (curing reagent was 4-5%), and the residuary amount of hydrogen bonding was still high after curing. With the high novolac composition (80%),their final cured samples should be still miscible. However, the techniques used in refs 7 and 8, FT-IR and DSC, are not able to provide sufficient information about the phase structures for a cross-linked blend. In this paper, our attention is focused on highly crosslinked novolac/PMMA blends (20wt % of curing reagent relative to novolac component is used) over the whole composition range. The blends after heating at 200 "C without curing reagents were also examined to distinguish between the effects of heating and cross-linking. Highresolution solid-state NMR, Fourier transform infrared (FT-IR), and differential scanning calorimetry (DSC) techniques have been used in this study. The highresolution solid-state NMR technique, which is particularly powerful for cross-linked polymers when many other techniques fail to give clear results, is able to provide clear information of phase structures on scales from the molecular level to 20-30 nm.*12 We aim to obtain clear evidence to show whether microphase separation or even macrophase separation occurs for the blends after the novolac component is fully cured. The effects of the novolac composition of the blends, the amount and distribution of intermolecular hydrogen bonding, and the curing condition are also discussed. The results are helpful in understanding the curing effect on miscible novolac blends, which is of both theoretical and practical interest.
Experimental Section Samples. The conventionalnovolac sample used, Novolac-A (N-A),contains 0.15% of free phenol. Ita chemical structure is of phenol rings bridge-linked randomly by methylene groups with 25% orthoortho, 53% ortho-para, and 22% para-para methylene bridges as obtained from the 13C solution NMR spectrum. Poly(methy1methacrylate) (PMMA) was obtained from Aldrich Chemicals,with a molecular weight of 120 OOO and aglasstransitiontemperature (T,) of 114OC. N-A/PMMA blends were prepared by the solution-castingmethod. The twopolymers 0 1994 American Chemical Societv
4920 Zhang and Solomon
were mixed in 2-butanone (1% (w/v)) at room temperature accordingto designed compositions. The mixing solutions were stirredfor 6-8 h, and the solvent was allowed to evaporate slowly at room temperature for about 24 h. The blends were then dried at 50 "C for 2 days under vacuum and annealed at 140 " C for 2 h. N-A/PMMA blends were cured with 20 w t % hexamine (hexamethylenetetramine)relative to the composition of N-A component in the blends (the ratio of phenolic ring of novolac to the methylene of hexamine is 1.0/1.1). The curing was performed in a tube oven for 10 min, 30 min, 1 h, 2 h, and 3-4 h at 160 and 190 "C, respectively. Samples without hexamine were heated at 200 "C as well under the same conditions. After curing or heating, the samples were quenched by liquid nitrogen to freezetheir phase structuresand dried at 50 "C under reduced pressure for 2 days. Heating and curing were restricted to 200 "C to avoid thermal degradation. High-Resolution Solid-state NMR. High-resolutionsolidstate NMR experiments were carried out at room temperature on a Bruker MSL400 spectrometer at resonance frequencies of 400 MHz for proton and 100 MHz for carbon-13, respectively. High-resolution lacNMR in the solid state was performed by the cross polarization (CP)/magicangle sample spinning (MAS) techniquetogether with the high-powerdipolar decoupling (DD) technique. The 90"pulse was of 5.0 gs, and the contact time of CP was set at 1.2 ms for all experiments. The rate of MAS was 15-19 kHz formeasuring'W spectraand 8.5-10 kHz for measuring relaxation times. The chemical shift of all l3C spectra was determined by taking the carbonyl carbon of solid glycine (176.03 ppm relative to TMS) as an external reference standard. *HTI, and TIvalues were observedthrough high-resolved'9c resonances by the CP techniquewith the pulse sequencesreported elsewhere.9 FT-IRand DSC Measurements. FT-IRstudieswere carried out on a BIO-RAD FTS-65A infrared spectrometer with the conventionalKBr disk method. All spectra were signalaveraged by 64 scans at a resolution of 4 cm-1 at room temperature. Band decomposition of the carbonyl group of the PMMA component was done by the BANDFIT software, with a fitting error of 1.01.5%. The glass transition temperatures were measured by differential scanning calorimetry (DSC) on a Perkin-Elmer DSC7 apparatus. The measurementwas made on ca. 8-10 mg samples after quickly cooling specimens to room temperature following the first scan. For samples cured by hexamine for a short time (e.g., 10and 30 min),the first scan was stopped at 130"C to avoid further curing during the measurement. The scan rate was 20 "C/min within a temperature range of 20-240 "C. The Tgvalues were recorded as the midpoint of the heat capacity change, and the reproducibility of T,values was within 1 "C.
Results and Discussion Miscibility of Novolac/PMMABlends. 13CCP/MAS spectra of Novolac-A, PMMA, and their blends are shown in Figure 1. The four peaks of N-A correspond to the methylene carbons (33 ppm), the ortho-unsubstituted carbon in phenol rings (116 ppm), the hydroxyl-substituted (C-OH) carbon at 151 ppm, and the other carbons in phenol rings (129 ppm), respectively. The five carbon resonances of PMMA are due to the a-CH3 (16.3 ppm), the quaternary carbon (44.9 ppm), the OCH3 and the CH2 (51.9 and 56.0 ppm), and the carbonyl (C=O) carbon at 177.0 ppm, respectively. Our attention is focused on the carbonyl carbon of PMMA and the hydroxyl-substituted carbon of N-A. As shown in Figure 2, the C=O carbon resonance of the PMMA component shifts downfield as N-A increases in the blends, corresponding to the formation of intermolecular hydrogen-bonding interaction. Resonances in a hydrogen bond would experience a different chemical environment with a relative weak magnetic shielding. Modification of bond angles and variations in intrachain distances of nearest neighbors are also possible after the formation of hydrogen bonding.13 Such shortrange effects cause a downfield shift. The shift of the
Macromolecules, Vol. 27, No. 18, 1994 PMMA
15/85
A 30170
An I
350
200
.
,
100
.
,
0
50
ppm
Figure 1. 13C CP/MAS NMR spectra of Novolac-A, PMMA, and their blends. 1 ao
P
150
'Oo0
7
L I 0
20
40
60
BO
100
3 0 J 0
0
I
I
20
C-OH
,-. 40
I
60
, . 80
I
100
W% (Novolac-A)
Figure 2. Chemical shifts and line widths of the hydroxylsubstituted carbon and the carbonylcarbon resonancesof uncured Novolac-A/PMMAblends. C=O carbon in the 80/20 blend is 2.3 ppm relative to that of neat PMMA, which is comparable to that of other hydrogen-bonded miscible blends.lPl7 The line width of the carbonyl carbon resonance also broadens as the N-A component increases due to a distribution of various bonded and nonbonded C=O resonances. The behavior of the C-OH carbon resonance of N-A is similar. However, ita largest downfield shift is just 1.4 ppm, and the line width does not increase linearly with increase of the PMMA component. A broad line can be observed only when PMMA is dominant in the blends. This is attributed to its complicated hydrogen-bonding behavior. The hydroxyl group can act as both a hydrogen-bonding donor and an acceptor; intramolecular interactions either between two hydroxyl groups located in two methylenebridged phenol rings or between those a t any other two nearby phenol rings could occur in pure N-A. The downfield shift of the C=OH carbon resonance indicates that the intermolecular hydrogen bonding between N-A and PMMA is stronger than the intramolecular bonding within N-A, which is similar to that of poly(vinylpheno1) in its miscible
Macromolecules, Vol. 27, No. 18, 1994
Phase Structures of Cross-Linked Novolac/PMMA Blends 4921 Novolac-A/PMMA=50/50 Blend
A
1 9O0C/2h-10%H
I \
190°C/10mln-10%H
16 O"C/lOm i n- 10% H
1850
1750 1700 Wavenumbers
1800
1650
Figure 3. FT-IRspectra of the carbonyl stretchingrange of the Novolac-A/PMMA blends observed at 25 O C : (A) Novolac-A/ PMMA = 80/20, (B)65/35, (C) 50/50, (D)30/70, (E) 15/85, and pure PMMA. 200
180
140
160
120
100
ppm
Figure 5.
lacCP/MAS spectra of Novolac-A/PMMA= 50150
blend after heating and curing.
70
0
20
W%
40
60
80
100
(Novolac-A)
Figure 4. Glass transition temperatures of uncured NovolacA/PMMA blends. FT-IR spectra provide a quantitative description of the specific intermolecular hydrogen-bonding interaction between the two components in a blend.'* As shown in Figure 3, the absorption band of the carbonyl group of pure PMMA appears at 1731 cm-l. Increasing the N-A content in the blends causes the intensity of the band at 1699-1704 cm-l to increase significantly, which is attributed to the carbonyl group hydrogen-bonded to the hydroxyl group of N-A. The 30 cm-l shift of the carbonyl stretching mode in N-A/PMMA blends is comparable with the values (20-30 cm-9 observed in other miscible novolac blends1*31s*7*8 and poly(vinylpheno1)b l e n d ~ . ~ ~A9 ~single 0 glass transition temperature was indeed observed for all N-A/PMMA blends by DSC (Figure 4). Our PMMA sample with a higher Tg(114 "C) relative to atactic PMMA (106 OC) should correspond to a high syndiotactic PMMA. Thus, the curve of Tgvs composition of the blends shows a similar S-shape to that of novolac/syndiotactic PMMA blends2 as described by a common expression for the Tg of polymer blends with the contribution of intermolecular hydrogen bonding. It was suggested that more facile contact could occur between syndiotactic PMMA and novolac chains.2 Phase Structures of Novolac/PMMA Blends after Curing. After Novolac-A/PMMA blends were cured by hexamine at 160 and 190 OC, significant changes were
noticed in l3C CP/MAS spectra as shown for example in Figure 5 for the 50/50 blend. The chemical shifts and line widths of the C=O and the C-OH carbon resonances of N-A/PMMA blends after curing and heating are summarized in Table 1. After curing, the relative intensity of unsubstituted orthocarbon in the phenolic ring (116 ppm) decreases because partial curing reactions occur at the ortho-unsubstituted position of phenolic r i n g ~ . ~ Note l-~~ that the C=O carbon of the PMMA component shifts upfield and its line width becomes narrower. However, only a slight change can be noticed for blends heated at 200 "C without hexamine. These results indicate that the decrease of the amount of intermolecular hydrogen bonds between the two polymers after curing is mainly due to the effect of cross-linking and not heating. However, the C=O carbon resonance in the cured blends still locates downfield relative to pure PMMA, especially when N-A is rich. This suggests that a considerable amount of intermolecular hydrogen bonds still remains in these fully cured blends. The shifts and broadening of the C-OH in the cured blends are due to the dissociation of the hydrogen bonding and the formation of various chemical structures during curing.21 The further broadening of the C-OH resonance in the cured blends is consistent with the curing of N-A in the PMMA matrix resulting in heterogeneities of molecular environment. The curing effect on these miscible blends is also apparent in the FT-IR and DSC results. FT-IR spectra of N-A before curing, after heating a t 200 "C without hexamine, and after curing at 190 "C with 10% of hexamine are shown in Figure 6. Heating N-A at 200 "C without hexamine causes no change in the absorption bands. After curing at 190 "C with hexamine, the hydroxyl stretching band of N-A becomes sharper and appears at a higher wavenumber. Note that the C-OH carbon of N-A also shifts 0.4 ppm upfield in I3CCP/MAS spectra after curing (Table 1). These results indicate that the curing also breaks the intra- and intermolecular hydrogen bonding
4922 Zhang and Solomon
Macromolecules, Vol. 27,No. 18, 1994
Table 1. lF Chemical Shifts. (ppm) and Line Widthss (in Parentheses (Hz))of the C=O of PMMA and the C-OH of Novolac-A in the Blends C=O
PMMA, before heating PMMA. 200 OC/2 h Novolad-A, before curing
C-OH
177.0 (440) 177.0 (440) 151.4 (590) 151.0 (600)
Novolac-A, 190 OC/2 h/lO% H
Novolac-A/PMMA = 80120 blend before curing 200 OC/l h/no H 200 "C/1 h/16" H 200 '(73.5 h/16% H Novolac-A/PMMA = 65/35 blend before curing 200 OC/2 hino H 160 "C/lO min/l3% H 190 OC/lOmin/l3% H 190 "C/2 h/13% H Novolac-A/PMMA = 50/50 blend before curing 200 "C/2 hino H 160 min/lO% H 160 "C/2 h/10% H 190 "CilO min/lO% H 190 OC/2 h/10% H Novolac-A/PMMA = 30/70 blend before curing 200 OC/1 h/no H 160 OC/lO min/6% H 160 "C/2 h/6% H 190 "C/2 h/6% H Novolac-A/PMMA = 15/85 blend before curing 200 OC/2 h/no H 160 "CilO min/3% H 160 'C/2 h/3% H (I
179.3 (710) 179.0 178.5 (490) 178.3 (480)
151.6 (640) 151.8 (670) 151.6 (630) 151.1 (610)
178.6 (670) 178.6 (630) 178.0 (560) 177.5 (450) 177.1 (450)
152.1 (700) 151.9 (760) 152.1 (710) 151.8 (710) 151.3 (670)
177.5 (620) 177.8 (600) 177.3 (590) 177.0 (520) 177.2 (490) 177.1 (460)
152.2 (700) 152.1 (710) 152.3 (780) 151.6 (770) 151.8 (700) 151.4 (710)
177.2 (560) 177.2 (490) 177.2 (470) 177.2 (450) 177.0 (450)
152.5 (720) 152.8 (750) 152.5 (720) 152.1 (710) 152.0 (720)
177.2 (490) 177.2 (470) 177.3 (450) 177.2 (450)
152.8 (900)
-"--" 120
80
4 0
160
Temperature
200
("C)
Figure 7. DSC curves of Novolac-A (A) before curing, (B)after heating at 200 OC/2 h without hexamine, and after curing with 10% hexamine at (C) 160 "C/10 min, (D) 160 OC/30 min, (E) 160 "C/l h, and (F) 200 "C/2 h.
1
152.8 (870) 152.8 (760)
Accuracy of measurementa. chemicalshift,f0.2 ppm; line width,
f 2 0 Hz.
~
i
200"CIZh-noH
I-J i
i
before curing
ll
I
11
I 1
I
I
I
3600 3400 3200 3000 2800 Wavenumbers
I 4000
I
I
3500
3000
I
I
2500 2000 Wavenumbers
1
1500
io00
500
Figure 6. FT-IR spectra of Novolac-A before curing, after heating at 200 "C/2 h without hexamine, and after curing at 190 "C/2 h with 10% hexamine.
within N-A chains. The formation of a cross-linked network causes N-A chains to lose steric advantage and to be less flexible and hence to reduce hydrogen bonding. A new band appears at 1655-1625 cm-l, corresponding to amine intermediates formed during curing. The formation of intermediates such as benzoxazine21reduces the number of phenolic hydroxyl groups and thus also reduces the number of hydrogen bonds. Figure 7 shows the change of the Tgbehavior for N-A after curing. The heating resulb in an increase in the Tgdue to the loss of free phenol and other low-molecular-weightvolatile compounds which act as plasticizers. Heating a t 200 "C for 2 h increases its Tg from 74to 81 "C. However,the transition is still apparent. After curing N-A at 160 "C for 10 min, the range of the glass transition temperature becomes very broad, ca. 80-
i
I
1850
1750 1650 Wavenumbers
Figure 8. FT-IR spectra of the hydroxyl and carbonylstretching ranges of Novolac-A/PMMA = 65/35 blend before heating (A) and after heating at 200 "C without hexamine for (B) 10min, (C) 30 min, (D)1 h, and (E) 2 h.
130 "C, which is consistent with a broad T,at 124 O C for novolac after curing by 4% of hexamine.8 Further curing causes the glass transition of N-A to disappear. This is a common phenomenon for highly cross-linked polymers. In the curing of novolacs with a high amount of curing reagent, cross-linking could occur at most phenolic rings. Thus, the cross-link density could be high enough to restrict segmental motions and result in the Tgbehavior dieappearing. FT-IFtspectra of the 65/35blend after heating and curing are shown in Figures 8 and 9. Heating blends at 200 O C without hexamine only decreases the band intensity of the hydrogen-bonded C=O slightly. A single apparent Tgwas observed for all of these blends. After curing the blends with hexamine at 190 "C, the band intensity of the hydrogen-bonded C=O (at 1702 cm-') decreases signifi-
Phase Structures of Cross-Linked Novolac/PMMA Blends 4923
Macromolecules, Vol. 27,No. 18, 1994
Blend
Novolac-A/PMMA=80/20
-3
B0 u
80
60
U 0
7
40
0
a
4
20
I
0
1
2
3
4
0
1
2
3
4
Healing Time (h)
I
lA
3600 3400 3200 3000 2800
1750 1650 Uavenumbers
1850
Wavenumbers
Figure 9. FT-IR spectra of the hydroxyl and carbonylstretching ranges of Novolac-A/PMMA = 65/35 blend before curing (A) and after curing at 190 "C with hexamine for (B)10 min, (C) 30 min, (D) 1 h, and (E) 2 h.
Figure 11. Amount of the hydrogen-bonded carbonyl groups and Tgvalues for Novolac-A/PMMA= 80/20 blend after heating at 200 "C without hexamine ( 0 )and after curing at 200 O C ( 0 ) with hexamine. The filled circles indicate the Tg of pure Novolac-A after heating at 200 OC without hexamine. Novolac-A/PMMA=65/35 Blend 1
1 - 0 8
0
1
2
4
I
4
3
0
0
.
1
7
2
Heating Time (h)
Figure 12. Amount of the hydrogen-bonded carbonyl group and Tgvalues for Novolac-A/PMMA= 65/35 blend after heating at 200 "C without hexamine ( 0 )and after curing at 190 ( 0 )and 160 "C (A) with hexamine. The fiied circles indicate the Tgof pure Novolac-A after heating at 200 "C without hexamine.
4 0
,
I
I
80
120
160
Temperature
Novolac-A/PMMA=50/50
Blend
1
0
200
("C)
Figure 10. DSC curves (second run) of Novolac and NovolacA/PMMA blends after curing with hexamine: (A) Novolac-A/ PMMA = 15/85,160 "C/3 h; (B)30/70,190 OC/2 h; (C) 50/50, 190 "C/2 h; (D) 65/35, 190 "C/2 h; (E) 80/20, 200 OC/3 h; (F) Novolac-A, 200 "C/2 h. cantly with increased curing time. The hydroxyl group band also shifts to higher wavenumber. These results are consistent with the 13C CP/MAS NMR spectra, which indicate the dissociation of intermolecular hydrogen bonding after curing. Since the driving force for the miscibility of N-A/PMMA blends is the intermolecular hydrogen bonding, the residual amount of the hydrogen bonds should be critical for the miscibility and phase structures of the cured blends. A Tgbehavior can only be noticed for PMMA-rich blends after curing by hexamine as shown in Figure 10. When N-A is more than 65 % ,the glass transition of the PMMA component in the blends was not observed. The amount of hydrogen-bonded C=O groups and Tg values of all N-A/PMMA blends after heating and curing
0
2
3
4
1
2
3
4
Heating Time (h)
Figure 13. Amount of the hydrogen-bonded carbonyl group and Tgvalues for Novolac-A/PMMA= 50/50 blend after heating at 200 "C without hexamine (0) and after curing at 190 ( 0 )and 160 "C (A)with hexamine. The filled circles indicate the TIof pure Novolac-A after heating at 200 OC without hexamine. are shown in Figures 11-15. The amount of hydrogenbonded C=O in most blends decreases by 4-12s after heating at 200 "C. In the 15/85 blend it decreases significantly after heating. A single Tgvalue is observed for all of these blends. As the heating time increases, the Tgvalues become somewhat higher relative to those before curing, possibly because the Tgof the N-A component
4924 Zhang and Solomon
Macromolecules, Vol. 27, No. 18, 1994
Novolac-A/PMMA=30/70 B l e n d
*7-----7
140
Table 2. lH TlPValues. __
i
I Novolac-A 200 "C/2 hino H Novolac-A 200 OC/2 h/lO% H PMMA 200 "Ci2 h/no H
b
'
-
2
Y
r
4
.
2
3
"
Heating Time (h)
Figure 14. Amount of the hydrogen-bonded carbonyl group and TBvalues for Novolac-A/PMMA= 30/70 blend after heating
and after curing at 190 (0) and at 200 "C without hexamine (0) 160 OC (A)with hexamine. The filled circles indicate the Tgof pure Novolac-A after heating at 200 "C without hexamine. Novolac-A/PMMA=l5/85 B l e n d L7
______
-1
Novolac-AiPMMA = 80120 200 OC/2 h/no H 200 OC/l h/16% H 200 "Ci3.5 h/16% H Novolac-AiPMMA = 65/35 200 OC/2 hino H 160 OC/l h/14% H 190 "C/2 h/14% H Novolac-A/PMMA = 50150 200 "C/2 hino H 160 OC/2 h/lO% H 190 "C/1 h/lO% H Novolac-A/PMMA = 30170 200 OC/l hino H 160 OC/2 h/6% H 190 OC/2 h/6% H Novolac-AiPMMA = 15/85 200 OC/2 h/no H 160 "Ci2 h/3% H
(ms)for Novolac-A/PMMA Blends PMMA Novolac-A PMMA 178 129 116 52 45 Novolac-A ppm ppm ppm ppm ppm 33ppm 8.4 8.3 8.4 8.2 12.5
8.3
8.5 12.7 12.6
9.1 8.9 9.0 8.9 10.0 9.9 10.0 10.1 10.3 10.8 11.0
9.1 10.2 10.3
9.6 10.9 11.7
9.8 9.7 9.6 9.8 10.2 9.8 10.8 11.2 10.2 10.4 11.9 11.6
9.5 9.8 10.3
11.2 12.0 13.1
11.1 10.8 11.0 11.3
10.9 10.6 11.0
11.3 12.0 13.3
11.6 11.5 11.5 11.6 11.2 10.9 11.9 11.9 10.0 9.9 13.3 13.5
11.7 12.8
11.9 10.8
10.6 10.4 11.6 12.2 10.8 10.9 12.8 13.0
12.0 12.1 12.7 12.8
The accuracy of the measurements is f3-7 % .
60:
,
,
,
,
Heating Time (h)
Figure 15. Amount of the hydrogen-bonded carbonyl group and T pvalues for Novolac-A/PMMA= 15/85blend after heating at 200 "C without hexamine (0)and after curing at 160 "C (A) with hexamine. The filled circles indicate the Tg of pure Novolac-A after heating at 200 "C without hexamine. increases after loss of low-molecular-weight materials during heating. For PMMA-rich blends after heating at 200 "C, their Tgvalues tend to be the same as that of neat PMMA. However, this does not indicate phase separation of the blends, because we did not observe a Tgaround 80 "C correspondingto the N-A component. When the blends are cured by hexamine, the amount of hydrogen-bonded C=O group initially decreases dramatically as curing time increases. After curing for 30 min to 1 h, the value does not decrease with further curing. Note that for all blends cured at 160 "C, the amount of intermolecular hydrogen bonds is higher than for those cured at 190 "C. The Tg behavior of the cured blends shows composition dependence. When N-A is rich, no apparent Tgbehavior can be observed even after 10 min of curing. When the PMMA composition reaches 50% and over, a glass transition is observed at 117-120 OC,which is somewhat higher than that of PMMA heated at 200 "C for 2 h (Tgof 115 "C). These results indicate that heating the blends at 200 "C without hexamine only slightly destroys intermolecular hydrogen bonding and the blends are still miscible. The significant decrease of the intermolecular hydrogen bonds between N-A and PMMA is mainly due to a curing effect, but not heating. DSC results suggest that the N-A-rich blends after curing may still be homogeneous. Note that no glass transition of the PMMA component was observed for cured N-A-rich blends. Because the curing rate is fast
and the rate of phase sepration is slow, the PMMA chains could be frozen in the N-A cross-linked network after curing. Therefore, segmental motions of PMMA chains are restricted, and no Tgbehavior can be observed. When PMMA is rich, it is just possible to form a pure PMMA phase or a PMMA-rich phase after curing the N-A component. Then a clear Tg behavior of the PMMA component can be detected. However, these results are not sufficient to give an insight into the phase structures of these fully cured blends. Moreover, it is very necessary to know the scale of domain sizes of these blends after a large amount of intermolecularhydrogenbonding is broken during curing. If there are some PMMA chains in the N-A cross-linked network, it is important to know whether the PMMA chains distribute uniformly in the network. Also we need to know whether PMMA-rich blends are partially miscible or totally phase-separated after curing. The phase structure of a blend is critical for its properties, because different domain sizes could bring about totally different mechanical properties. Usually miscible blends provide average properties of the component polymers, while microphase-separatedblends still keep the properties of single components. If macrophase separation occurs, the whole material may have no useful mechanical properties at all. High-resolution solid-state NMR is able to provide more detailed information about the phase structures. lH TI, values of Novolac-A/PMMAblends after heating and curing were observed through high-resolution l3C resonances as listed in Table 2. For pure amorphous N-A and PMMA, 1H T1, values observed through all 13C resonances of each polymer are identical, respectively, and the curing of Novolac-A does not change ita TI,value. After heating N-AIPMMA blends at 200 OC without hexamine, the TI, values observed from the resonances of both N-A and PMMA components are identical in all blends, and these values are also quite close to the protonaveraged 21' , values of pure N-A and PMMA after heating at 200 OC/2 h. These results indicate that a fast spin diffusion occurs among all protons in these blends which
Phase Structures of Cross-Linked NovolacIPMMA Blends 4925
Macromolecules, Vol. 27, No. 18, 1994 Table 3. 1H TIValues. (e) for Novolac-A/PMMA Blends
PMMA Novolac-A
Novolac-A 190 "(212h/10% H 65/35 blend 190 "C/2h/13% H 50/50 blend 190 "C/1h/10% H 30/70 blend 190 "C/1h/6% H 15/85 blend 160 OC/2 h/3% H PMMA 200 "C/2h a The
178 ppm
PMMA 129 116 52 45 Novolac-A ppm ppm P P ~P P ~ 33ppm 1.14 1.13 1.15
0.74
0.79 0.79 0.75 0.76
0.78
0.78
0.83 0.84 0.77 0.76
0.83
0.74
0.78 0.77 0.73 0.74
0.74
0.75 0.74 0.72 0.73
0.65
0.67 0.67
accuracy of the measurements is *3-7 7%.
averages out the whole relaxation process. Thus, the domain size of these blends is smaller than the spindiffusion path length within 1H TIPtimes, ca. 2-3 nm. The results also indicate that the increased Tgvalues of the blends after heating at 200 OC, even for the 15/85 blend, are not due to a phase separation. The situation of cured blends is complicated. Note that the Tlp values observed through both N-A and PMMA components are identical only for the 80120 blend after curing at 200 OC/1 h. For the other cured blends, the observed Tlpvalues of the two components are different to some extent. This indicates that the domain size of these cured blends is larger than a scale of 2-3 nm. The spin diffusion within the TIPtime cannot efficiently average out the whole relaxation process. Table 3 lists the 'H TI values observed through high-resolution 13C resonances for N-A/PMMA blends after fully curing with hexamine. Except for the N-AIPMMA = 50150 blend, the TI values observed from the two components in the cured blends are quite close, and even identical for the 65/35 and 15/85 fully cured blends. Even for the 50/50 fully cured blend, the TI values of the two components also tend to be close to each other. Thus, the domain sizes of most fully cured blends are on scales of 20-30 nm, which is the characteristic maximum spin-diffusion path length during lH TI times. Note that the TI,,values of the PMMA component in N-Arich blends or those of the N-A component in PMMA-rich blends are shorter or longer than those of the pure polymers, respectively. Miscible phases should still exist in these cured blends which act as adhesive phases; thus, macrophase separation does not occur after curing. For novolac-rich blends after curing, a semi-IPN may occur; novolac forms a highly cross-linked network through the whole material, while PMMA chains distribute in the network as domains. When novolac is very rich, the domain size of the PMMA component is even smaller than 2-3 nm. When PMMA is rich, the small amount of novolac makes it impossibleto form a cross-linked network through the whole material. In these cases, PMMA is the matrix while cross-linked novolac chains distribute in the PMMA phase. The slightly higher Tgvalues observed for cured PMMA-rich blends (117-120 "C) could be due to the existence of cross-linked novolac chains in the PMMA matrix. Most fully cured Novolac-A/PMMA blends appear as microphase-separated systems with domain sizes around a scale of 20-30 nm. For the 50/50 cured blend, it is possible to have both novolac-rich and PMMA-rich phases in the system. The scale of such phases is relatively large. The amount of intermolecular hydrogen bonds between the two polymers and the scales of domain sizes for
Table 4. Amounts of Intermolecular Hydrogen Bonding and Domain Sizes of Novolac-ARMMA Blends after Heating and Curing hydrogen domain size (nm) mole bonding" (%) 1H TlP 1H TI ratio Novolac-A/PMMA = 80/20* 81/19 13 2-3 200 "C/3h/l6% H Novolac-A/PMMA = 65/35 66/34 200 "C/2h/no H 21 2-3 >20-30 190 "C/1h/10% H Novolac-A/PMMA = 30170 31/69 200 "C/1h/no H 13
