Two glass transitions of polyurea networks: effect of the segmental molecular weight

Author(s)

Marius Reinecker, Viktor Soprunyuk, Martin Fally, Antoni Sanchez-Ferrer, Wilfried Schranz

Abstract

Polymer–nanoparticle composites (PNCs) play an increasing role in

technology. Inorganic or organic nanoparticles are usually incorporated

into a polymer matrix to improve material properties. Polyurea is a

spontaneously occurring PNC, exhibiting a phase segregated structure

with hard nanodomains embedded in a soft (elastically compliant) matrix.

This system shows two glass transitions at T_g1 and T_g2.

It has been argued that they are related to the freezing of motion of

molecular segments in the soft matrix (usual polymer α-glass transition

at T_g1) and to regions of restricted mobility near the hard nanodomains (α′-process) at T_g2,

respectively. We present detailed dynamic mechanical analysis (DMA)

measurements for polyurea networks with different segmental lengths l_c (2.5, 12.1, 24.5 nm) of the polymer chains, i.e. different volume fractions ϕ_x (0.39, 0.12, 0.07) of the hard domains. The two glass transitions show up in two distinct peaks in tanδ at T_α and T_α′.

Analysing the data using a Havriliak–Negami term for the α- and

α′-relaxation, as well as Vogel–Fulcher dependencies for the

corresponding relaxation times, it is found that the α-glass transition

at T_g1 increases strongly (up to ΔT = 70 K) with increasing ϕ_x, whereas the α′-transition at T_g2 remains unchanged. At ϕ^c_x ≈ 0.19 the two curves intersect, i.e. T_g1 = T_g2. This value of ϕ^c_x

is very close to the percolation threshold of randomly oriented

overlapping ellipsoids of revolution with an aspect ratio of about 1:4–1:5.

We therefore conclude that around 19% of the hard nanodomains polyurea

changes from a system of hard nanoparticles embedded in a soft matrix (ϕ_x ≤ ϕ^c_x) to a system of soft domains confined in a network of percolated hard domains at ϕ_x ≥ ϕ^c_x.

Organisation(s)

Physics of Functional Materials

External organisation(s)

Eidgenössische Technische Hochschule Zürich

Journal

Soft Matter

Volume

10

Pages

5729-5738

No. of pages

10

ISSN

1744-683X

DOI

https://doi.org/10.1039/c4sm00979g

Publication date

2014

Peer reviewed

Yes

Austrian Fields of Science 2012

103023 Polymer physics, 103018 Materials physics

Keywords

ASJC Scopus subject areas

Condensed Matter Physics, Chemistry(all)

Portal url

https://ucris.univie.ac.at/portal/en/publications/two-glass-transitions-of-polyurea-networks-effect-of-the-segmental-molecular-weight(ef1f10bc-0b2f-4b94-8d84-c63130eef2ec).html