Photon Factory Activity Report 2016 #34 (2017)

BL-10C/2016G538

Nonisothermal Crystallization of a Defect-Free Polymer Network1

Shintaro Nakagawa1,*, Xiang Li2, and Mitsuhiro Shibayama2
Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505,
Japan
2
Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba,277-8581, Japan

1 Introduction
It is well known that polymer networks can crystallizein spite of the fact that the translational motion of theconstituting chains is limited by the network connectivity.
Although there have been a number of reports on thecrystallization of polymer networks [1, 2], crystallization
might be affected by the topological defects such as loops
and dangling chains that are inevitably incorporated in therandomly-crosslinked networks. Recently, Sakai andcoworkers [3] found that a polymer network almost free ofsuch topological defects could be prepared by end-linkingof four-armed poly(ethylene glycol) (Tetra-PEG). In thisstudy, we investigated the crystallization kinetics andstructure formation during nonisothermal crystallization ofa Tetra-PEG network to reveal the effects of network
connectivity on polymer crystallization.
2 Experiment
ATetra-PEG
network(Fig. 1) was prepared by endlinking between two Tetra-PEG
prepolymers having aminegroups (TAPEG) and activatedester groups (TNPEG) at thechain-ends. Note that the1:
Schematicsolvent used in the end-linking Fig.
oftheprocess
wascompletely illustration
removed from the sample prior samples.
to measurements. The molecular weight of each arm of theprepolymers was 5.0×103 g mol−1. Bulk TAPEG was also
used as a control (Fig. 1). Time-resolved SAXS/WAXDmeasurements were conducted at the beamline BL-10C in
KEK-PF. SAXS and WAXD profiles were collectedduring cooling from 80 °C (well above the typical melting
temperature of PEG) to 10 °C at the rate of −5 °C.
3 Results and Discussion
Fig. 2 shows the development of the crystallinity of PEG
chains obtained from time-resolved WAXD profiles of thenetwork and prepolymer
during cooling. The onset
temperatureofcrystallization of thenetwork is higher than thatoftheprepolymer.
However, the slope of thecurve, which roughlyreflects the crystal growthrate, is lower for thenetwork than for the Fig. 2: Crystallinity of PEG
prepolymer. These facts chains during cooling.

suggest that network connectivity reduces the mobility of
PEG chains and eventually accelerates the crystal
nucleation but decelerates the subsequent crystal growth.
Moreover, the final crystallinity of the network at 10 °C issignificantly lower than that of the prepolymer, probably
due to the reduced degree of conformational freedom of
PEG chains imposed by network connectivity.

Fig. 3: SAXS profiles for the prepolymer (a) and network(b) during cooling.
Fig. 3 shows the SAXS profiles of the prepolymer andnetwork during cooling. As soon as the crystallization
begins, a peak emerges in both samples, indicating theformation of periodically stacked lamellar crystals.
However, the change in the primary peak position q* is
different between the prepolymer and network. q* of theprepolymer is almost constant at 0.035 Å−1 whereas that of
the network gradually shifts to higher q during thenonisothermal crystallization process. The shift of q* forthe network suggests that the initially formed crystals are
thicker than the thermodynamically stable one and hence
slowly become thinner as the crystallization proceeds. It
can be concluded that the thinning of the initially formed
lamellar crystals is significantly retarded in the network
compared to that in the prepolymer because of the reduced
chain mobility by network connectivity.
Acknowledgement
This work was supported by a Grant-in-Aids for
Scientific Research from the Ministry of Education,
Culture, Sports, Science, and Technology (No. 16H02277).
The authors thank Dr. Nobutaka Shimizu at KEK-PF for
his help during the experiments at BL-10C.
References[1] W. S. Lambert et al., Polymer 35, 1809 (1994).
[2] M. Shibayama et al., Polymer 35, 2944 (1994).
[3] Sakai, T. et al., Macromolecules 41, 5379 (2008).
* snaka@iis.u-tokyo.ac.jp