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 Tg1 and Tg2.
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 Tg1) and to regions of restricted mobility near the hard nanodomains (α′-process) at Tg2,
respectively. We present detailed dynamic mechanical analysis (DMA)
measurements for polyurea networks with different segmental lengths lc (2.5, 12.1, 24.5 nm) of the polymer chains, i.e. different volume fractions ϕx (0.39, 0.12, 0.07) of the hard domains. The two glass transitions show up in two distinct peaks in tanδ at Tα and Tα′.
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 Tg1 increases strongly (up to ΔT = 70 K) with increasing ϕx, whereas the α′-transition at Tg2 remains unchanged. At ϕcx ≈ 0.19 the two curves intersect, i.e. Tg1 = Tg2. This value of ϕcx
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 (ϕx ≤ ϕcx) to a system of soft domains confined in a network of percolated hard domains at ϕx ≥ ϕcx.
Originalsprache | Englisch |
---|---|
Seiten (von - bis) | 5729-5738 |
Seitenumfang | 10 |
Fachzeitschrift | Soft Matter |
Jahrgang | 10 |
Ausgabenummer | 31 |
DOIs | |
Publikationsstatus | Veröffentlicht - 2014 |
ÖFOS 2012
- 103023 Polymerphysik
- 103018 Materialphysik