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Scott M. Husson, Ph.D. -- Research Activities
Thrust 1: Surface Modification
The overall objective of this thrust area is to change the chemical and physical
properties of polymeric and inorganic surfaces in rational ways that improve
their performance. The first project uses controlled graft polymerization;
the second project involves small molecule chemistries.
Surface Modification of Microporous PVDF Membranes by ATRP
Overview: Growing polymer chains from a surface imparts new interfacial properties
by masking the underlying substrate. Generally speaking, the chemical nature
and spatial arrangement of the chains can be manipulated via synthesis to yield
systems with tailored chemical and physical properties. Our group uses a relatively
new technique called atom transfer radical polymerization (ATRP) to grow ultrathin
(< 10 nm) polymer films from inorganic and organic surfaces. ATRP is a so-called
controlled radical polymerization technique, which means that termination events
are minimized to yield uniform chain growth.
Opportunity: There is significant commercial interest in developing surface
treatment methods to modify membrane properties post casting. In fact, membrane
surface
modification now appears to be equally important to the membrane industry as
membrane material and process development. Ideally, the manufacturer would cast
a ‘base’ membrane that could be modified to have chemical and physical
properties for a specific application.
Research activities and findings: A goal of this work is to examine whether ATRP
can be used to simultaneously change the surface chemical functionality, tune
average pore-size, and narrow pore-size distribution of a microporous membrane.
Figure 1 shows SEM images of a commercially available poly(vinylidenedifluoride)
membrane that was modified using graft polymeriza¬tion of poly(2-vinylpyridine)
(PVP) from the surface of the membrane. Polymerization time was used as the independent
variable to manipulate the amount of grafted PVP on the membrane surface. By
changing polymerization time, the average surface pore diameter decreased from
1.11 ?m to 0.98 ?m. Equally important, the initially broad pore-size distribution
became narrower following polym¬erization. The pore diameter polydispersity
(PDP) decreased from 2.05 to 1.44 following polymerization for 24 hours. (A PDP
value of 1.0 means that all pores are equally sized.) This result is important
for membrane applications in chromatography, because broad pore size distributions
lead to inefficient membrane utilization caused by premature solute breakthrough.
In addition to changing the membrane physical properties, addition of PVP transformed
the membrane chemical properties.
Figure 1. SEM surface images of a PVDF membrane prior to polymerization (left)
and after 24 hours of poly(2-vinylpyridine) growth (right). Magnification is
2000 X.
Surface Amidation to Reduce Coefficient of Friction of Ethylene-co-Acrylic Acid
Film
Overview: Polyolefin film surfaces are tacky and exhibit a high coefficient
of friction (COF). To reduce COF values, long-chain primary amides are
often blended
into polyolefins prior to extrusion. Because these amide additives are
chemically incompatible with the polymer, they migrate or “bloom” to
the film surface over time, where they act as slip agents to lower COF.
Opportunity: Because the amide additives are not bonded chemically to the film
surface, additive loss is a concern. From a processing point of view, additives
are removed over the course of repeated film-metal contacts, thereby increasing
COF. From a healthcare perspective, amide additives have been shown to migrate
from plastics into food products, where they act as sleep-inducing compounds.
A direct, permanent modification to the film surface would eliminate these concerns.
Research activities and findings: Working with another group at Clemson and a
researcher from Cryovac Division of Sealed Air Corporation, we developed an approach
to amidate the surface of ethylene-co-acrylic acid (EAA) copolymer films. Surface
amidation was achieved by direct conversion of EAA carboxylic acid groups and
via grafting of the amino acid intermediate, 12-aminododecanoic acid, followed
by amidation of its acid group. Both surface amidation schemes reduced the kinetic
COF from 0.30 to 0.15 ~ 0.18. Repetitive COF testing revealed that amide-modified
EAA films maintained low COF values that were independent of the number of COF
test runs.
Figure 2 shows an unexpected result: Analyzing scanning probe microscopy (SPM)
data, we discovered that a majority of the COF reduction appeared to be due to
changes in surface roughness of the films upon solvent exposure only (i.e., without
amidation). This result suggests that a simple solvent treatment may impart more
permanent slip properties to a surface than blending a migratory slip agent.
Figure 2. COF as a function of the number of repetitive COF tests
(a) amide-modified EAA films and (b) solvent treated EAA films.
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