J . Phys. Chem. 1989, 93, 331 1-3313
TABLE VI: Parameters Involved in Eq B uarameters m‘/kcal A mol-l c’lkcal class of compd alkali-metal oxides ( M 2 0 )
(a eq 4)
moP
986.7 (2.44) -21.3
alkali-metal sulfides (M2S)
1044.4 (2.66)
9.2
alkaline-earth-metal oxides (MO)
1540.0 (2.97)
-30.8
alkaline-earth-metal sulfides (MS)
1920.0 (5.8)
-95.2
alkali-metal selenides (M2Se)
1042.9 (2.66)
22.7
alkaline-earth-metalselenides (MSe) 1787.5 (4.36) -54.3
dev in AHLO’, kcal mo1-I max = 1.5 av = 1.1 max = 3.0 av = 1.5 max = 4.7 av = 3.2 max = 4.0 av = 1.9 max = 4.3 av = 3.7 max = 1.9 av = 1.6
Discussion The empirical, two-parameter equations illustrated above describe an accurate correlation for determining AHLO or AHLO’ and hence AfHofrom the lattice parameter for cubic crystals. Alternatively, if we know AfHo,then it is possible to use these equations to determine the nearest-neighbor distance with better than 0.5% accuracy. In certain cases such as the oxides of alkaline-earth metals the maximum deviation obtained is more than 2 kcal, and these may be attributed to experimental errors involved in the measurements of AHLO. Although a linear relationship is illustrated mainly for groups IA and IIA metal compounds having cubic structures, we have found that halides of any metal cations and ammonium with cubic structures can be related linearly. The results are given in Table
3311
11. However, it is not possible to relate compounds of other metals with those of the main-group metal compounds, although both may have the same cubic structures. For example, NaCl type MnO does not fit in the line of group IIA metal oxides (see Figure 4). Similarly, CsCl type TlBr does not fall on the line for the group IA metal halides (see Figure 1). If one examines the enthalpies of atomization (values are given in Table I) of the alkali-metal halides or hydrides as functions of rMx,they show the qualitative but not the quantitive behavior observed for the lattice enthalpies. While these do not rule out a covalent description of the M X salts, it does tell us that no simple, general covalent model will describe the entire group.
Acknowledgment. This work has been supported by a grant from the National Science Foundation (CHE-87- 14647). Registry No. LiF, 7789-24-4; LEI, 7447-41-8; LiBr, 7550-35-8; LiI, 10377-51-2;NaF, 7681-49-4; NaC1,7647-14-5; NaBr, 7647-15-6; NaI, 7681-82-5; KF, 7789-23-3; KCI, 7447-40-7; KBr, 7758-02-3; KI, 768111-0; RbF, 13446-74-7;RbCl, 7791-11-9; RbBr, 7789-39-1; RbI, 779029-6; CsF, 13400-13-0; CsCI, 7647-17-8; CsBr, 7787-69-1; CsI, 778917-5; LiH, 7580-67-8; NaH, 7646-69-7; KH, 7693-26-7; RbH, 1344675-8; CsH, 13772-47-9; LiLi, 14452-59-6; NaNa, 25681-79-2; KK, 25681-80-5; RbRb, 25681-81-6; CsCs, 12184-83-7; TIBr, 7789-40-4; SrCI,, 10476-85-4; BaCI2, 10361-37-2; CaF,, 7789-75-5; SrF,, 778348-4; BaF2, 7787-32-8; AgI, 7783-96-2; AgBr, 7785-23-1; AgCI, 778390-6; AgF, 7775-41-9; NH4F, 12125-01-8;NH,CI, 12125-02-9;NH4Br, 12124-97-9; NH,I, 12027-06-4; Li20, 12057-24-8; Na20, 1313-59-3; K,O, 12136-45-7; Rb20, 18088-11-4; Li2S, 12136-58-2; Na2S, 131382-2; K2S, 1312-73-8; Rb2S, 31083-74-6; Liz%, 12136-60-6; Na2Se, 1313-85-5; K2Se, 1312-74-9; MgO, 1309-48-4; CaO, 1305-78-8; SrO, 1314-11-0; BaO, 1304-28-5; MIS, 12032-36-9; Cas, 20548-54-3; SrS, 1314-96-1; Bas, 21109-95-5; Case, 1305-84-6; SrSe, 1315-07-7; BaSe, 1304-39-8; MnO, 1344-43-0.
Phase Transition of Aqueous Solutions of Poly(N-isopropylacrylamide) and Poly(N-isopropylmethacrylamide) Shouei Fujishige,* Research Institute f o r Polymers and Textiles, 1-I Higashi, Tsukuba 305, Japan
K. Kubota, College of Technology, Gunma University, 1-5 Tenjin, Kiryu 376, Gunma, Japan
and I. Ando Tokyo Institute of Technology, 2-12 Oh-okayama, Meguro, Tokyo 152, Japan (Received: July 19, 1988; In Final Form: September 14, 1988)
When aqueous solutions of well-fractionated poly(N-isopropylacrylamide) samples are heated, the polymer molecular dimensions change abruptly at a critical temperature (-32 “ C ) , followed by aggregation of individual polymer chains dispersed in a state of globular particles to give an optically detectable phase transition. The transition occurs independently of either the molecular weight of the polymer (5 X lo4 to 840 X lo4) or its concentration (0.01 to 1 wt %). This behavior is reminiscent of the thermal denaturation of proteins in aqueous medium.
To elucidate the role of water molecules in thermal denaturation of biological polymers, various properties of aqueous polymer
solutions have been studied by many investigator^.'-^ Among those, aqueous solutions of relatively simple synthetic polymers
(1) Kauzmann, W. Nature 1987, 325, 763-764. (2) Klotz, E. M. Fed. Proc. 1965, 24 (Suppl. No. IS), S24-33. (3) Horne, R. A,; Almeida, J. P.; Day, A. F.; Yu, N. J . Colloid Interface Sci. 1971, 35, 77-84.
(4) Heskins, M.; Gillet, J. E. J . Macromol. Sci., Chem. 1968, A2, 144 1-1 455. ( 5 ) Molyneux, P. Water-Soluble Synthetic Polymers: Properties and Behaoior; CRC Press: Boca Raton, FL, 1983, 1984; Vols. I, 11.
0022-3654/89/2093-33 1 1$01.50/0
0 1989 American Chemical Society
Fujishige et al.
3312 The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 Poly (NIPAM) poly INIPMAM) 0 a
methylcellulose
E
(25cps)
0 0 I n 42
m
u W
i E 5
t
1.J / y i ,
I
I
0 a
1
/
/
4
I '
/
A more detailed but still qualitative study on such an aqueous solution exhibiting the phase separation upon heating was conducted on poly(N-isopropylacrylamide) [hereafter abbreviated as poly(NIPAM)] by Heskins et al." using only one unfractionated polymer sample. They obtained a phase diagram reminiscent of a rounded cloud point behavior which is characterized as a typical one for a system exhibiting the lower critical solution temperature (LCST). In our previous paper,6 we established an effective procedure for molecular weight fractionation of poly(N1PAM) and then derived the intrinsic viscosity-molecular weight relationships by using these fractionated samples. in this paper, we present more detailed data on the phase transition and the dynamic and static light scattering data measured on an extremely dilute solution in the vicinity of the phase transition temperature of poly(N1PAM) in an aqueous solution. CH3
I
Figure 1. Phase diagram of 1 wt % aqueous solution of poly(N-isopropylacrylamide) in comparison with those of 1 wt 3'% poly(N-isopropylmethacrylamide) and of 0.5 wt % methylcellulose (commercially
CCH2-
-+CH2-C-f;r
I
c=o
available as Methocel). H-N
that undergo phase separation a t about 30-40 OC on heating have attracted particular attention because of a certain similarity to the denaturation of proteins in the aqueous phase. KlotzZ found that an aqueous solution of poly(vinylmethy1oxazolidinone) exhibits phase separation on heating at about 40 OC. According to Horne et al.,3 an aqueous atactic poly(viny1 methyl ether) solution also exhibits similar phase separation on heating at about 34 OC. Some interesting features were presented in these papers, but their detailed analysis has not fully been developed because of a lack of data on the molecular characteristics of the polymer samples used.
CH+
I
I I H
H3C-C-CH3
poly ( N I PAM )
I I H-N I H3C-C-CH3 I H
c=o
polyf N I PMAM)
The phase transition was traced by monitoring the transmittance of a 500-nm light beam through I-cm sample cells a t different temperatures on a specially constructed spectrophotometer.' The rates of heating up and of cooling down of the sample cells were adjusted at 1 OC/min. (effects of concentration)
(effects of molecular weight)
1 4 [:::I 9 3O'C
0'2546
25'C
~~o~~
0.077%
25'C
0.048%
25.C
""1) 25'C
[M=8,400,000] 0.033%
I O0%
I
transmittance
0%
Figure 2. Molecular weight dependence and concentration dependence of the phase transition of aqueous poly(N1PAM) solutions: (-) (- - -)
cooling down.
heating up,
Phase Transition of Poly(N1PAM) and Poly(N1PMAM) A typical example of the phase diagram of aqueous poly(N1PAM) solution is shown in Figure 1, in comparison with that of poly(N-isopropylmethacrylamide) [hereafter abbreviated as poly(NIPMAM)] and that of methylcellulose in aqueous solution. It is seen that the phase transition of aqueous poly(N1PAM) solution is quite sensitive, reversible, and reproducible to the thermal stimulation, in contrast to that of other polymers. The trace of the phase transition of aqueous poly(N1PMAM) solution is accompanied by a significant retardation for the rate of cooling, as indicated by a dotted line, which might be attributed to effects of a restricted free rotation on the total conformation of this polymer due to the presence of the a-methyl group in the main chain unit structure. A striking feature shown in Figure 2 is that the phase transition of this aqueous solution of poly(N1PAM) takes place as the temperature reaches 31 "C, contrary to the previous work,4 independently of either the molecular weight of the polymer (5 X IO4 to 840 X IO4) or its concentration (0.01 to 1 wt %). The heat of the phase transition of aqueous poly(N1PAM) solution determined by differential scanning calorimetry was endothermic of the order of 10 cal/g of polymer. In accordance with the above, the dynamic as well as static light scattering data on an extremely dilute aqueous poly(N1PAM) solution at temperatures from 15 to 32 "C have demonstrated that a characteristic dimensional change of the chain molecule occurs in aqueous solution. It is seen in Figure 3 that, especially in the vicinity of the phase transition temperature, an abrupt conformational change occurs from a state of well-solvated random coils a t lower temperature to a state of tightly packed globular particles at higher temperature and that this process is completely reversible as a function of temperature. This type of transition phenomenon, particularly of aqueous poly(N1PAM) solution, might be considered as an initial stage of the phase separation and one of typical experimental evidence to confirm a coil-globule transition of polymer chains which has long been predicted theoretically by Ptitsyn et aI.* By taking account of such striking transition phenomenon as a characteristic feature of aqueous poly(N1PAM) solution, it would be drawn such a picture as the coil-globule transition of the chain molecule takes place solely depending on the temperature of aqueous polymer solution a t the initial stage of the phase ( 6 ) Fujishige, S. Polym. J . 1987,19, 297-300. (7) Itoh, S.,manuscript in preparation. (8) Ptitsyn, 0. B.; Kron, A. K.; Eizner, Yu. Ye. J . Pdym. Sci., Pnrt C 1968,3509-3517.
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3313 150 nn
100 m
0 : root-mean-square ( static light
50 nm
'
20
radius of gyration scattering )
hydrodynamic radius ( dynamic light scattering )
25
30
32
temperature, ["CI
Figure 3. Effects of temperature on the dimensional parameters of ply(N1PAM) (Kfw= 8400000) in an extremely dilute aqueous solution (concentration: 1.264 X g/g).
separation. This is then followed by the onset of aggregation of individual chain molecules due mainly to the intermolecular interaction between the hydrophobic groups distributed on the surface of the resulting globular particles of the polymer in aqueous medium. This interpretation would reasonably be expected to explain the fact, presented in our previous paper: that above 30 "C capillary shear flow in an usual viscometer induces the formation of tiny fibrillar textures, which have a lifetime of the order of several minutes and then disintegrate into a homogeneous solution but which disturb viscosity measurements of the aqueous solutions above the critical temperature. This rather peculiar phase transition phenomenon observed on an aqueous solution of a synthetic polymer with relatively simple structure may be used as a model system for simulating the thermal denaturation of biological polymers and might be helpful for further understanding of more complicated processes involved in many living species. Registry No. Poly(NIPAM), 25 189-55-3
