Macromolecules 1995,28, 6963-6969
6963
Relaxations in Poly(di-n-alkyl and diisoalkyl itaconates) R. Diaz Calleja,*JL. Gargallo? and D. Ftadi6* Departamento de Termodinamica Aplicada, UPV, 46071 Valencia, Spain, and Departamento de Quimica Fisica, Facultad de Quimica, Pontificia Universidad Catdlica de Chile, Casilla 306, Santiago 22, Chile Received March 31, 1995; Revised Manuscript Received June 26, 1995@ ABSTRACT: A systematic study about the viscoelastic and dielectric relaxations was performed for a family of poli(di-n-alkyland diisoalkyl itaconates): poly(dimethy1itaconate)(PDMI),poly(diethy1itaconate) (PDEI),poly(di-n-propylitaconate)(PDPI),poly(di-n-butylitaconate)(PDBI),poly(diisopropy1itaconate) (PDIPI),and poly(diisobuty1 itaconate) (PDIBI). Three dielectric relaxation processes, labeled as a,p, and y were observed in all cases. Nevertheless, in some polymers a poor resolution of the peaks is observed and only small indications of the phenomena can be seen. In the case of PDPI and PDBI measurements at very low temperatures were performed and dielectric absorption processes are found at those temperatures. Behavior similar to that observed for dielectric relaxations is found for the mechanical one. The temperature at which the a relaxation, associated with the glass transition temperature, is present diminishes as the length of the n-alkyl side group increases. This behavior is similar to that reported for poly(n-alkyl methacrylates). p relaxation is only detected at some frequencies, and it is more easily detected in terms of E" or e'' than in terms of tan 6. This relaxation is less pronounced than in the case of poly(n-alkyl methacrylates). An important difference between these polymers and poly(n-alkylmethacrylates)is the existence of a new relaxation process labeled as y relaxation between -40 and -120 "C. This relaxation is very sensitive to the dryness of the sample or to the presence of molecules of low molecular weight in the matrix.
Introduction
nal rotations is higher due to the configurational versatility. Therefore, the number of relaxations should Esterification of itaconic acid (2-methylenesuccinic be higher than in the case of poly(methacry1ates) or acid) is a very interesting way to obtain monomers poly(acry1ates). In previous papers reported by Cowie which present an attractive array of possible structural et al.3@b924 and Diaz Calleja et al.8J3,25 several prominent variations. Mono- and diesterification of itaconic acid relaxation phenomena were described. can be carried out, obtaining monomers and polymers In this context it is interesting to refer to the work having either one or two of the carboxyl groups esterilisted as ref 24 which is an important precedence to the fied in each repeating unit.1,2 Monoesters can also be present paper. In that work, only viscoelastic relaxation selectively esterified in order to obtain diesters with two measurements were reported and the majority of them different side groups, specifically methyl alkyl diesters were performed in braids, taking into account that the can be ~ b t a i n e d .The ~ variation of the chain length of experimental equipment was a TBP (torsion braid the ester group provides several different polymer series pendulum). Using this machine, it is possible to obtain with very interesting properties.2~3,4a,5,6a,7-9 Poly(monoonly relative values for the modulus and loss angle, itaconates) may be considered as typical comblike although the position of the peaks, and therefore the polymers depending on the length of the side chain.7c activation energies of the corresponding processes, can The effects of the length of the side chain and the be ,detected with accuracy. presence of the carboxylic group have been taken into account to explain the particular conformational behavOn the other hand, dielectric measurements are ior of some poly(monoitaconates).7COn the other hand, commonly performed in a wide frequency range which poly(di-n-alkyl itaconates) obtained from diesters of includes the range corresponding to the mechanical itaconic acid and lower unbranched alcohols show ones, and in many cases the dielectric spectra are more important changes in both solution and the solid state resolvable than the mechanical one. p r o p e r t i e ~ . l !The ~ ~ *thermal ~~ stability and dielectric and Taking into account this background, this work was viscoelastic properties of members of several series of performed considering the properties of the correspondpoly(itaconates) and copolymers containing itaconate ing family of poly(n-alkyl and isoalkyl methacryunits have been assessed by several a ~ t h o r s . ~ ~ J ~l a- ~t e~ ~ ) which ~ ~ - can ~ ~be considered as precursors of the Information covering the physical proper tie^,^^^,^^-^^ poly(diitac0nates) studied in the present work. dilute solution behavior,16-18 polyelectrolyte and ion Our previous background in poly(mono- and diesters) exchange p r o p e r t i e ~ , ~ ~biological J ~ J ~ , ~and ~ antitumor derived from itaconic a ~ i d ~allows J ~ ,us~ to ~ perform a behavior,21s22and response to high-energy radiationz3 systematic study of the family of poly(dialky1itaconates) of these materials is available. with nonaromatic, linear or branched substituents in In the case of poli(di-n-alkyl itaconates), there is an the side chain in order to get a generalization about the important increasing of the steric hindrance because of relaxation properties in this important class of polythe presence of two side groups per repeating unit. mers. Therefore, motions corresponding to these groups will In order to reach these objectives, a systematic study be inhibited as a whole due to the presence of adjacent about the viscoelastic and dielectric relaxation propersubstituents. However, the possibility of partial interties for a family of poly(di-n-alkyl and diisoalkyl itaconates), poly(dimethy1 itaconate) (PDMI), poly(diethy1 + UPV. itaconate) (PDEI), poly(di-n-propyl itaconate) (PDPI), Pontificia Universidad Cat6lica de Chile. Abstract published in Advance ACS Abstracts, August 1,1995. poly(di-n-butyl itaconate) (PDBI), poly(diisopropy1ita-
*
@
0024-9297/95/2228-6963$09.00/00 1995 American Chemical Society
Macromolecules, Vol. 28, No. 20, 1995
6984 Diaz Calleja et al. conate) (PDIPI), and poly(diisobuty1 itaconate) (PDIBI), will b e carried out.
F' L
Experimental Section Synthesis of Monomers and Polymers. Diitaconates were prepared by conventional acid-catalyzed esterification of itaconic acid (1mol) with the corresponding alcohols (3-4 mol), using p-toluenesulfonic acid in toluene.24 The pure monomers were obtained by repeated distillation of the crude product under reduced pressure; purity was confirmed by 'H and I3C NMR and 1R spectro~copy.~~ Free radical polymerization of monomers was carried out in bulk at 323 K using a,a'-azobis(isobutyronitri1e)(-0.3 mol %) as initiator under N2 (polymerization time, between 40 and 70 h depending on the monomer; conversion -70%). The reaction mixture was dissolved in chloroform, and the polymer was isolated by precipitation. Polymers were purified by repeated dissolution and reprecipitation before drying in vacuo at 323 K. Fractionation and Characterization. Polymers were fractionated by solubility using benzene/methanol as the solvent precipitant pair. Several fractions were obtained, and fractions with weight average molecular weights (M,)ranging between 2 x lo5 and 3 x lo6 were selected for the present work, with polydispersity indices (MJMn) of 1.25-1.35. Molecular weights of the samples were determined by static light scattering (SLS) and size exclusion chromatography (SEC) in THF using a Dawn-F light scattering instrument from Wyatt Technology, containing 15 permanently mounted detectors, using the equipment in the flow mode and a Perkin-Elmer high-performance liquid chromatograph (HPLC) with a 6000 psi pump, a Perkin-Elmer differential refractometer Model LC25, an injector of 175 pL, and three Waters Associated Ultrastyragel columns (lo3,lo4, and lo5 in series. Refractive index increments (dnldc) of the samples were determined using a Wyatt Optilab 903 interpherometer from Wyatt Technology Corp. using a P2 flow cell of 2 mm. Viscoelastic Measurements. A dynamic thermomechanical analyzer DMTA-MARK I1 was used for the relaxation measurements in the flexural mode. Samples were always prepared by molding and heating and then dried in a vacuum oven until constant weight. The drying process took 25-30 days. In some cases probes containing remnent solvent (THF) were used in order to analyze the effect of the presence of low molecular weight substances on the relaxation properties. In the majority of the samples the dryness temperature was up to 140 "C. A double cantilever or single cantilever measurement form was used in a random way according to the amount of polymer used. No significant differences were observed using these two procedures for the same material. Five frequency measurements were used: 0.1,0.3,1,3, and 10 Hz, although in some cases 0.3, 1, 3, 10, and 30 Hz were used. The temperature range was between -140 or -120 "C and 20 deg above the value of the peak corresponding to the a relaxation at 1 Hz. In all cases the heating rate was 1"C/ min. After the drying process, the samples were fragile; therefore, in order t o avoid the breaking of the probes during the clamping in the DMTA, samples were slightly clamped and then a fast scan at 5 "C/min until a temperature where the polymer is rubber-like was carried out. After this process, probes were carefully and slowly tightened until room temperature. Dielectric Measurements. The dielectric relaxation measurements were carried out in a dielectric analyzer DEA 2970 from TA Instruments using molded and dried disklike probes in a way similar to the probes for mechanical measurements. In this kind of measurement it was also necessary t o heat the probes and at the same time to reduce the thickness between the electrodes in order to stick the polymer to the electrodes. Only by this way was it possible to obtain a reasonable value of E', indicating good contacts between electrodes and sample, with reproducible results considering the experimental error. The temperature measurement range was, in the majority of the cases, similar t o that for the mechanical measurements and 14 frequencies were used between 1 and 3 x lo4 Hz.
IO'
loo
t lo"
10'
-120
-80
-40
0
LO
120
80
T
OC
Figure 1. Storage and loss tangent for ( 0 )PDMI, ( x ) PDEI, (0)PDPI, and (A) PDBI as a function of the temperature at 1 Hz. In the case of PDPI and PDBI measurements were performed from -250 "C in a homemade cell into an Edwards helium cryostat controlled by a ICT 4 unit. The measurement equipment was then a bridge GENRAD 1620, and the measurement range was from 200 to lo4 Hz. The measurements performed with this equipment were in good agreement with those performed with the Analyzer DEA 2970. In this case the measurements were carried out by sticking the probes to the hot electrodes. Stabilization of the system a t room temperature took several hours before the measurements. Samples were cooled at -250 "C, and then measurements were performed at intervals of 10 deg. A stabilization time was required at each temperature. For the lower temperature this time can be about 2 h, but as the temperature increases, the stabilization time was shorter.
Results Mechanical Spectroscopy. Figure 1 represents the storage modulus and the loss tangent at 1 Hz for PDMI, PDEI, PDPI, and PDBI as a function of t e m p e r a t u r e . Clearly one c a n observe the relaxation p e a k associated with the glass transition temperature (T&, together with the effect of the length of the side chain on the relaxation process. The values of the t e m p e r a t u r e for tan 6,, (1Hz)for the four polymers considered are 95, 71, 58, and 30 "C, respectively. These values c a n be correlated with the corresponding values for the four first poly(methacry1ates) of the series (105, 74,34, 18 "C). As c a n be observed in Figure 2, the effect of the decrease of Tgwith the side c h a i n is more sensitive in the case of poly(methacry1ates) than in the case of poly(itaconates). It seems indicative that in spite of the double esterification the puckering effect is more imp o r t a n t in the case of poly(itaconates) than in poly(methacrylates). This result is opposite to that reported b y Cowie et al.24w h e r e the internal plastification due to the effect of the side c h a i n s seems t o be similar in b o t h polymers. In o u r case the internal plastification is n o t more pronounced than in the case of poly(methacrylates). A t lower temperatures, one cannot detect viscoelastic activity defined in terms of the loss tangent. Using the
Macromolecules, Vol. 28, No. 20, 1995
380
-
360
-
2-40
-
320 300
Poly(di-n-alkyland diisoalkyl itaconates) 6965
0 0
8 e
-
0
'"v -r
0
0
I
I
I
1
I
1
2
3
4
Number of C atoms in the side chain Figure 2. Dependence of the maxima of tan 6 (at 1 Hz)on the n-alkyl length. E"(Pa)
4 lo2 -100 -60 -20 20 60 100 140
-140
T
Figure 4. Storage, loss modulus, and loss tangent for ( 0 )PIP1 and (0)PIBI as a function of the temperature at 1 Hz.
tan s
n
(OC)
I
1
tan 8
18
1
I '
PDMI X
PDEI
0
POPI
0,1
0,Ol
A PDBI 1 Hz I
I
1
I
I
1
I
T OC Figure 3. Loss modulus for the same polymers under the same conditions as in Figure 1: ( 0 )PDMI, ( x ) PDEI, (0)PDPI, (A) PDBI. modulus E" (Figure 3) to represent the viscoelastic loss, the results seem to be better. The existence of two weak relaxations in the proximity of -80 t o -60 "C and around -20 to f 2 0 "C can be assumed. The last ones appear overlapped with the a peak. The first polymer of the series is that which presents the best defined spectrum. Nevertheless, the resolution is better than that reported by Cowie et al. in Figure 2 from ref 24 about the same problem. In that work they found a broad B relaxation that, according to our results, shows structure. In any case it is more diffused than the p relaxation of PMMA. For this reason the relaxation a t temperatures -20 and +20 "C will be labeled as the B relaxation, and that corresponding to the lower temperature as y accordingly t o the common terminology. For polymers containing branched side chains, Figure 4 shows the variation of the modulus E', E , and the
1
1
1
1
-120 -80 -40
1
1
0
1
1
40
1
80
1
~
~
1
0,Ol
120
T OC Figure 6. Comparison between the viscoelastic tan 6 for the dry and swollen samples of PDPI and PDIBI at 1Hz. loss tangent at 1 Hz for PDIPI and PDIBI in the temperature range under study. Two relaxations can be observed, where the most prominent is the a relaxation, assigned as in the former case t o the glass transition temperature. The temperatures of the maxima in tan 6 a t that frequency for the a relaxation are 98 and 104 "C, respectively. Therefore, the Tgincreases as the volume of the side chain increases. This result is in good agreement with that observed for the corresponding family of poly(methacry1ates) (ref 27). For temperatures between -30 and -20 "C a secondary relaxation is observed which will be labeled as j3. This result can be better appreciated in the curves of E in Figure 4. It is noteworthy that in this case the Tg values are higher than that for PDMI, the first polymer of that series. This is in contrast with the
Macromolecules, Vol. 28, No. 20, 1995
6966 Diaz Calleja et al. 10'
7
tan 8
I
10-l
L
lo-*
-240
I
I
I
I
-140 -100 -60 -20
1
I
I
20
60
100
I
1
1 lo-'
140 180 T (OC)
Figure 6. Permittivity and loss tangent (lo2 Hz) for (0) PDMI, (0) PDEI, (v) PDPI, and (A) PDBI. The variation of tan 6 (10 kHz) at lower temperatures for PDPI and PDBI is also shown at the bottom (at 10 kHz). Symbols are the same as those of the main Figure. results observed for poly(methacrylates), where Tg for PMMA is higher than for methacrylic polymers with isopropyl and isobutyl groups. Moreover, Tg(PDIPI) < Tg(PDIBI) in contrast to what is observed in poly(methacrylates). This result would indicate that the puckering of the bulky side chain is larger in the case of poly(itaconates) than in poly(methacryltes), in spite of the presence of one ester group in the side chain per repeating unit. It is necessary t o remark that the comparison of the viscoelastic spectra of the dry probes with those obtained from samples without a dryness process shows a plastification effect on the a relaxation temperature, which is representative of the glass transition (Figure 5). This plastification clearly can be attributed t o the remainder solvent (THF) inside the matrix. However, it is surprising that this effect is also observed in sub-T, relaxations. This would indicate a specific type of polymer-solvent interaction in the glass state. This phenomenon would suppose a mechanical absorption due to a complex mechanism in which the solvent molecules should play some role. This behavior would be in agreement with the theory of J0ha1-i~~ in which a secondary process would be produced in specially disordered zones (clusters) of the matter. By this way it is not necessary to justify the relaxation by any molecular motion. Therefore, the subTgphenomena would also be attributed to low molecular weight substances. Dielectric Spectroscopy. The results corresponding t o dielectric relaxation measurements are more explicit than the mechanical ones. Figure 6 shows the data corresponding to E' and tan 6 at lo2 Hz for the four first members of the series of polytitaconates) between -120 and approximately 20 deg over the Tg. In these figures one can clearly observe the presence of a main relaxation that will be labeled as a and another relaxation at temperatures ranging from -40 "C for PDMI to -120 "Cfor PDBI. Near room temperature there are
-140
-xx)
-60 -20
20
60
100
KO T
180 (OC)
Figure 7. Permittivity and loss tangent (lo2Hz) for ( 0 )PDPI PDBI. and (0) 10'
In f
1210.4
5
4d 1 ; b R
; b ;
lo
1:
1:
;3
;4 111 ( K
I
Figure 8. Dielectric relaxation map, In f vs T-l (K), showing a,p, y , and 6 relaxational zones in the range of frequency and PDEI, (v)PDPI, ( A ) temperature studied: (0)PDMI, (0) PDBI, (@) PDIBI. PDIPI is omitted for clarity of the figure. More detailed information is given in Tables 1and 2. Because the a relaxation follows a WLF behavior instead of an Arrhenius type, the lines In f vs T-' are not extrictly straight lines and for this reason are not joined. evidences of another relaxation. All polymers show conductive effects above Tg. These are more important a t low frequencies, and in some cases, as in PDMI, they overlap the presence of the a peak a t those frequencies. The values at which the maxima in tan 6 as 1kHz are present are in good agreement with those obtained by mechanical measurements but about 20-26 deg higher. It is convenient to remember that the frequency of 1 kHz is lo3 times higher than those found for mechanical measurements. In the case of PDMI there is only 9 deg of difference, this polymer being an exception. These results confirm the conclusions obtained from Figure 2 dealing with the effect of internal plastification in poly(itaconates). Dielectric measurements from -240 "Cwere carried out in polymers containing longer side chains such as PDPI and PDBI. A relaxation about -200 "C for PDPI and others at -175 "C for PDBI can be observed at 10
Macromolecules, Vol. 28, No. 20, 1995
Poly(di-n-alkyl and diisoalkyl itaconates) 6967
Table 1. Temperature (“C) of the Peaks and Activation Energy (kJ mol-’)
a(€”) 100 f 1 84 f 1 82 f 1 48 f 1 98 f 1 114 f 1
PDMI
PDEI PDPI PDBI
PDIPI PDIBI
a(tan 6 )
Dielectric temp. at lo2 Hz, E , B(tan6 )
95 f 1 82 h 1 80 f 1 40 f 1 90 f 1 110 f 1
indica indic indic -30 f 1,91.3 f 0.5 possib1e indic
temp at 10 kHz, E , 6
Y
-64 f 1,41.5 f 0.5 -88 f 1,34.0 f 0.5 --BO, 28 f 1 --120 -20 f 1,62 f 1 -55 f 1,54 f 1
-196 f 2,14.0 f 1 -170 f 2,lB.B f 1 possible
Mechanical
temp at 1Hz Ehelax a PDMI
PDEI PDPI PDBI
PDIPI PDIBI a
E”/3 -25 -15 --20 --20 --20 --24
70 f 1 50 f 1 38 f 1 5 f l 72 f 1 180 f 1
E/y
d
--60 --60 --BO --BO
indic indic
-70
indic means that there are only indications.
Table 2. Temperature (“C) of the Peaks (f1“C) in Terms of the Dielectric Loss Moduli at 1 Hz PDMI PDEI PDPI PDBI PIP1 PIBI T(M”,,,)a 82 60 50 12 68 86 (M”max)/3 -0 -20 -52 T(M”,,,)y -98
