No abstract

The (specific) heat capacity signal from modulated temperature DSC can be used to measure the onset of phase separation in aqueous poly(N-vinylcaprolactam) (PVCL) solutions, showing a type I LCST demixing behavior. Quasi-isothermal measurements through the phase transition show large excess contributions in the (apparent specific) heat capacity, caused by demixing and remixing heat effects on the time scale of the modulation. These excess contributions and their time-dependent evolution are useful to describe the kinetics of phase separation and to follow the related morphology development. Partial vitrification of the polymer-rich phase and a higher polymer concentration in the solutions lower the rate of phase separation. The introduction of hydrophilic poly(ethylene oxide) grafts onto PVCL lowers the demixing temperatures and markedly enhances the rate of phase separation, for both aqueous PVCL solutions and PVCL hydrogels.

Thermo‐responsive graft copolymers have been synthesized based on a poly(N‐vinylcaprolactam) (PVCL) backbone and either hydrophilic poly(ethylene oxide) (PEO) or hydrophobic poly(tetrahydrofuran) (PTHF) side chains. The phase separation behavior of the graft polymers in water was studied by transmittance measurements and compared to that of the corresponding swollen segmented polymer networks and aqueous solutions of both polymers. The influence of the concentration and length of the grafts on the cloud point temperature (TCP) has been demonstrated. PVCL‐g‐PTHF copolymers have been synthesized by using the macromonomer technique, i.e. the radical copolymerization of VCL with a PTHF macromonomer. A special feature of these amphiphilic graft copolymers is their ability to stabilize aqueous emulsions below the TCP and to suddenly break them above the TCP. PVCL‐g‐PEO copolymers were prepared by a grafting onto method. First, succinimide groups were introduced in the backbone, to which amino terminated PEO chains were grafted in the second step. This leads to di‐hydrophilic copolymers that become amphiphilic after heating their aqueous solutions above the TCP.

Two‐ and three‐dimensional phase diagrams have been constructed for thermosensitive poly(N‐vinylcaprolactam)‐poly(ethylene oxide) (PVCL‐PEO) aqueous systems. Both solutions and swollen block copolymer networks have been investigated to elucidate the effect of the copolymer content and crosslinking density on their temperatures of phase separation (Tph.s.). The introduction of hydrophilic PEO into an aqueous solution of PVCL decreases its Tph.s.. This suggests that the strength of the hydrogen bonds within the thermoresponsive PVCL‐water system is weakened by the introduction of PEO that also interacts with water. Based on the DSC investigation of the swollen networks, it was found that the influence of PEO on the phase behavior of weakly crosslinked networks is comparable with that of PVCL‐PEO‐H2O solutions. For networks with a higher degree of crosslinking, the presence of the crosslinks is of major importance for the explanation of the Tph.s. location. This detailed phase analysis led to the proposal of an irregular water distribution in these swollen networks.