الفهرس 
HF TYPES AND APPLICATIONS
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Abstract
Hollow fiber membranes (HFMs) are currently used for a variety of applications such as
water treatment and desalination, biomedical applications (artificial internal organs) and
diversified biotechnological applications. In addition, the use of membranes for gas
separation has recently received increasing interest. On the other hand, numerous factors
affecting the desired membrane characteristics are interacting to set at the end the
requirements of a given application. Membrane drying represents a critical step affecting the
entire morphological, surface properties and mechanical characteristics of the fabricated
HFMs. The interaction of fiber spinning parameter with drying as a post-treatment has a
crucial impact on the functional aspects of the membrane.
In this work, the effect of microwave (MW) drying in different media comprising
distilled water, saline and MW in air have been investigated using polyethersulfone (PES)
HFMs. Investigations have been carried out using a domestic MW oven within the following
ranges: temperature (35-85˚C) and time duration up to 15 minutes. The following results have
been concluded:
The apparent morphological changes refer to a moderate dimensional stability as far as
internal and external fiber diameter. The maximum relative membrane thickness with respect
to their parent untreated samples are (-18.2 to 16%), (-34%), (-3.23 to 20.15%), (23.19%),
(75%) and (62.5%) for MW heating in distilled water, MW heating in sodium chloride
solutions, MW heating in lithium chloride solutions, MW heating in air at 55˚C, MW heating
in air at 85˚C and conventional heating in air media, respectively. In general, results for MW
heating in air and conventional air heating in oven indicated increased membrane thickness
for all investigated conditions. On the contrary, MW treatment in sodium chloride medium
manifests decreased thickness for all samples.
As far as surface roughness, obtained from atomic force microscopy (AFM)
measurements, is considered, the largest relative changes are (40.5%), from (-19%) to
(28.5%), (-79%) and (-71.5%) for MW heating in distilled water, MW heating in sodium
chloride solutions, MW heating in air at 85˚C and conventional heating at 150˚C in air media,
respectively.
Detailed analysis of the effect of MW heating on Young’s modulus, tensile stress at
break and break strain has shown moderate to significant changes as compared to their parent
untreated samples. The relative maximum increase of young’s modulus are (-4.8% to 19.7%),
iv
(-23.8% to 25.9%), (-47.44%), (-44%), (20.3%) and (36.1%) for MW heating in distilled
water, MW heating in sodium chloride solutions, MW heating in lithium chloride solutions,
MW heating in air at 55˚C, MW heating in air at 85˚C and conventional heating in air media,
respectively.
The maximum relative changes for tensile stress at break values are (-11.5% to 19.6%),
(-30.8% to 8.8%), (-32.6%), (-46.6%), (14.9%) and (27.9%) for MW heating in distilled
water, MW heating in sodium chloride solutions, MW heating in lithium chloride solutions,
MW heating in air at 55˚C, MW heating in air at 85˚C and conventional heating in air media,
respectively. Moreover, the absolute tensile stress at break values of the parent samples
(untreated) are 7.43, 7.62, 16.2, 16.2, 9.63 and 9.63 MPa for post-treatment using MW
heating in distilled water, MW heating in sodium chloride solutions, MW heating in lithium
chloride solutions, MW heating in air at 55˚C, MW heating in air at 85˚C and conventional
heating in air media, respectively. It is observed that values for tensile stress at break showed
positive increase for MW post-treatment in air and conventional air heating, where the largest
value is attributed to conventional air heating in oven.
The maximum relative changes for strain at break are (-16.9%), (-10.9% to 17%), (-49.2
to 21.13%), (-13 to 12.2%), (-8.6%) and (-13%) for MW heating in distilled water, MW
heating in sodium chloride solutions, MW heating in lithium chloride solutions, MW heating
in air at 55˚C, MW heating in air at 85˚C and conventional heating in air media, respectively.
Further, the absolute elongation percent at break for the parent samples are 52.1, 35.8, 50.5,
50.5, 45.4 and 45.4% for MW heating in distilled water, MW heating in sodium chloride
solutions, MW heating in lithium chloride solutions, MW heating in air at 55˚C, MW heating
in air at 85˚C and conventional heating in air media, respectively. With the exception of MW
post-treatment in saline solutions and in air at 55˚C, all other media depict negative impact on
maximum relative change values for tensile strain at break owing to the significant
contribution of ionic conduction mechanism in saline media, especially at 2% saline
concentration in MW.
These results confirm the effect of MW heating on morphological, surface roughness,
and mechanical characteristics of the PES HFMs manifesting minor and moderate relative
change values. It was also apparent that MW treatment in liquid media, whether water or
saline solution, enable utilization of both water dipole rotations and ionic conduction
mechanisms, respectively. Within the investigated range, MW treatment in saline media
shows significant contribution of the ionic conductance mechanism. Both, MW air heating
v
and conventional air heating emphasize the limited effect of ionic conductance mechanism
and the limited role of water rotational polarization mechanism since the only available water
is located within the pore and possibly the fiber lumen. Also, the results enable tuning of the
above mentioned characteristics with due consideration to the subsequent treatment
requirements or the specification of the final product.
Thus, a reliable working platform for MW heating has been developed as a choice matrix
for the specified processing requirements. Future research endeavors are still needed to
explore different saline media with variable concentrations and to explore other possible
benefits and drawbacks of this technology. Moreover, mixed organic/inorganic liquid media
should also be investigated to improve mechanical characteristics of MW heating as a post
treatment for hollow fiber membranes. Finally, the results confirm the future potential of MW
heating as a fast, clean technology to substantially replace conventional heating techniques.
Keywords: Hollow fiber membranes, Microwave heating, Post-treatment, Saline
solution, Mechanical properties, Surface roughness, Surface morphology.